Explicitly exclude unnecessary files in source distributions

[seisman]

Description of proposed changes

In #695, we switched our version manager from versioneer to setuptools-scm.

When building source distributions (sdist), the default behavior was only including files as listed in Python packaging guide. However, setuptools_scm's behavior is to include all files tracked by git (see https://github.com/pypa/setuptools_scm#file-finders-hook-makes-most-of-manifestin-unnecessary), and it seems there is no way to disable it (pypa/setuptools-scm#516).

So, instead of listing files that should be included in the source distributions, now we need to explicitly exclude some files from the source distributions.

Here is the list of files in pygmt v0.2.1 (https://pypi.org/project/pygmt/0.2.1/#files):

AUTHORS.md
CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE.txt
MANIFEST.in
PKG-INFO
README.rst
pygmt
pygmt.egg-info
setup.cfg
setup.py
versioneer.py

Here is the list of files using the MANIFEST.in file in this PR:

AUTHORS.md
CODE_OF_CONDUCT.md
LICENSE.txt
MANIFEST.in
PKG-INFO
README.rst
pygmt
pygmt.egg-info
pyproject.toml
setup.cfg
setup.py

You can run python setup.py sdist to build source distributions and check the tarball in the dist directory.

Fixes #904.

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 adding new functionality, add an example to docstrings or tutorials.

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]

Looks good. Really wish we could leave out the pygmt/tests/baseline folder full of PNGs to trim down on the package size, but that will come after resolving #963.

[seisman]

FYI, the tarball size can be reduced from 6.2 MB to 600 K if we can remove all baseline images. 😄