Allow printing show_versions() to in-memory buffer to enable testing

[weiji14]

Description of proposed changes

To increase code coverage of __init__.py, check the output of pygmt.show_versions() that is printed to an in-memory string buffer instead of stdout.

Adapted from https://github.com/xarray-contrib/xbatcher/blob/v0.2.0/xbatcher/tests/test_print_versions.py. Was looking through some code in xbatcher and noticed the neat print(..., file=io.StringIO) trick. Thanks @maxrjones!

Patches code coverage since #466

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[weiji14]

Code coverage report for this PR is at https://app.codecov.io/gh/GenericMappingTools/pygmt/pull/2399. The coverage for __init__.py has increased from 23.17% to 71.95% (+48.78%)

[seisman]

Looks great!

There is a unrelated CI failure, which is likely caused by recent upstream changes like GenericMappingTools/gmt#7295

____________________________ test_plot3d_projection ____________________________

args = ()
kwargs = {'data': array([[43.4847,  0.6227,  0.5309],
       [22.331 ,  3.7556,  0.3817],
       [40.8023,  5.5903,  0.7764],
 ...,  0.4305],
       [28.1125,  3.8456,  0.9338],
       [47.8333, -0.[722](https://github.com/GenericMappingTools/pygmt/actions/runs/4338387570/jobs/7575047135#step:17:723)5,  0.5969]]), 'region': [10, 70, -5, 10, 0, 1]}

    def wrapper(*args, **kwargs):
>       store.return_value[test_name] = obj(*args, **kwargs)

../../../../miniconda3/envs/pygmt/lib/python3.11/site-packages/pytest_mpl/plugin.py:107: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../pygmt/tests/test_plot3d.py:100: in test_plot3d_projection
    fig.plot3d(
../pygmt/helpers/decorators.py:818: in new_module
    return module_func(*args, **kwargs)
../pygmt/helpers/decorators.py:598: in new_module
    return module_func(*args, **kwargs)
../pygmt/helpers/decorators.py:738: in new_module
    return module_func(*args, **kwargs)
../pygmt/src/plot3d.py:242: in plot3d
    lib.call_module(
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <pygmt.clib.session.Session object at 0x14a06ae90>, module = 'plot3d'
args = '@GMTAPI@-S-I-D-V-T-N-000000 -Bag -Bzag -Ggreen -JR270/10c -Jz5 -R10/70/-5/10/0/1 -Ss1c -p225/30'

    def call_module(self, module, args):
        """
        Call a GMT module with the given arguments.
    
        Makes a call to ``GMT_Call_Module`` from the C API using mode
        ``GMT_MODULE_CMD`` (arguments passed as a single string).
    
        Most interactions with the C API are done through this function.
    
        Parameters
        ----------
        module : str
            Module name (``'coast'``, ``'basemap'``, etc).
        args : str
            String with the command line arguments that will be passed to the
            module (for example, ``'-R0/5/0/10 -JM'``).
    
        Raises
        ------
        GMTCLibError
            If the returned status code of the function is non-zero.
        """
        c_call_module = self.get_libgmt_func(
            "GMT_Call_Module",
            argtypes=[ctp.c_void_p, ctp.c_char_p, ctp.c_int, ctp.c_void_p],
            restype=ctp.c_int,
        )
    
        mode = self["GMT_MODULE_CMD"]
        status = c_call_module(
            self.session_pointer, module.encode(), mode, args.encode()
        )
        if status != 0:
>           raise GMTCLibError(
                f"Module '{module}' failed with status code {status}:\n{self._error_message}"
            )
E           pygmt.exceptions.GMTCLibError: Module 'plot3d' failed with status code 74:
Error:      plot3d [ERROR]: Western boundary is > 180 degrees from specified central meridian and thus your region is invalid
Error:      plot3d [ERROR]: General map projection error

../pygmt/clib/session.py:510: GMTCLibError
----------------------------- Captured stderr call -----------------------------
Error: [ERROR]: Western boundary is > 180 degrees from specified central meridian and thus your region is invalid
Error: [ERROR]: General map projection error

[seisman]

Looks great!

There is a unrelated CI failure, which is likely caused by recent upstream changes like GenericMappingTools/gmt#7295

____________________________ test_plot3d_projection ____________________________

args = ()
kwargs = {'data': array([[43.4847,  0.6227,  0.5309],
       [22.331 ,  3.7556,  0.3817],
       [40.8023,  5.5903,  0.7764],
 ...,  0.4305],
       [28.1125,  3.8456,  0.9338],
       [47.8333, -0.[722](https://github.com/GenericMappingTools/pygmt/actions/runs/4338387570/jobs/7575047135#step:17:723)5,  0.5969]]), 'region': [10, 70, -5, 10, 0, 1]}

    def wrapper(*args, **kwargs):
>       store.return_value[test_name] = obj(*args, **kwargs)

../../../../miniconda3/envs/pygmt/lib/python3.11/site-packages/pytest_mpl/plugin.py:107: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
../pygmt/tests/test_plot3d.py:100: in test_plot3d_projection
    fig.plot3d(
../pygmt/helpers/decorators.py:818: in new_module
    return module_func(*args, **kwargs)
../pygmt/helpers/decorators.py:598: in new_module
    return module_func(*args, **kwargs)
../pygmt/helpers/decorators.py:738: in new_module
    return module_func(*args, **kwargs)
../pygmt/src/plot3d.py:242: in plot3d
    lib.call_module(
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <pygmt.clib.session.Session object at 0x14a06ae90>, module = 'plot3d'
args = '@GMTAPI@-S-I-D-V-T-N-000000 -Bag -Bzag -Ggreen -JR270/10c -Jz5 -R10/70/-5/10/0/1 -Ss1c -p225/30'

    def call_module(self, module, args):
        """
        Call a GMT module with the given arguments.
    
        Makes a call to ``GMT_Call_Module`` from the C API using mode
        ``GMT_MODULE_CMD`` (arguments passed as a single string).
    
        Most interactions with the C API are done through this function.
    
        Parameters
        ----------
        module : str
            Module name (``'coast'``, ``'basemap'``, etc).
        args : str
            String with the command line arguments that will be passed to the
            module (for example, ``'-R0/5/0/10 -JM'``).
    
        Raises
        ------
        GMTCLibError
            If the returned status code of the function is non-zero.
        """
        c_call_module = self.get_libgmt_func(
            "GMT_Call_Module",
            argtypes=[ctp.c_void_p, ctp.c_char_p, ctp.c_int, ctp.c_void_p],
            restype=ctp.c_int,
        )
    
        mode = self["GMT_MODULE_CMD"]
        status = c_call_module(
            self.session_pointer, module.encode(), mode, args.encode()
        )
        if status != 0:
>           raise GMTCLibError(
                f"Module '{module}' failed with status code {status}:\n{self._error_message}"
            )
E           pygmt.exceptions.GMTCLibError: Module 'plot3d' failed with status code 74:
Error:      plot3d [ERROR]: Western boundary is > 180 degrees from specified central meridian and thus your region is invalid
Error:      plot3d [ERROR]: General map projection error

../pygmt/clib/session.py:510: GMTCLibError
----------------------------- Captured stderr call -----------------------------
Error: [ERROR]: Western boundary is > 180 degrees from specified central meridian and thus your region is invalid
Error: [ERROR]: General map projection error

Reported in GenericMappingTools/gmt#7295 (comment)