added netCDF4 requirement to failing tests

[feefladder]

• Closes Tests fail when installing h5netcdf without netcdf4 #4985 Tests fail when no SciPy installed #5565
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[feefladder]

Also there is an issue when:

cd xarray
pip install -e .
pip install h5netcdf pytest-xdist netcdf4
py.test -n 2 xarray/tests/test_backends.py

======================================================== FAILURES =========================================================
________________________________________ TestH5NetCDFFileObject.test_open_fileobj _________________________________________
[gw2] linux -- Python 3.9.5 /home/joeperdefloep/miniconda3/envs/xr-dev/bin/python3.9

self = <xarray.tests.test_backends.TestH5NetCDFFileObject object at 0x7f7e841a4220>

    def test_open_fileobj(self):
        # open in-memory datasets instead of local file paths
        expected = create_test_data().drop_vars("dim3")
        expected.attrs["foo"] = "bar"
        with create_tmp_file() as tmp_file:
            expected.to_netcdf(tmp_file, engine="h5netcdf")
    
            with open(tmp_file, "rb") as f:
                with open_dataset(f, engine="h5netcdf") as actual:
                    assert_identical(expected, actual)
    
                f.seek(0)
                with open_dataset(f) as actual:
                    assert_identical(expected, actual)
    
                f.seek(0)
                with BytesIO(f.read()) as bio:
                    with open_dataset(bio, engine="h5netcdf") as actual:
                        assert_identical(expected, actual)
    
                f.seek(0)
                with pytest.raises(TypeError, match="not a valid NetCDF 3"):
>                   open_dataset(f, engine="scipy")

/mnt/e/Git/xarray/xarray/tests/test_backends.py:2887: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/mnt/e/Git/xarray/xarray/backends/api.py:483: in open_dataset
    backend = plugins.get_backend(engine)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

engine = 'scipy'

    def get_backend(engine):
        """Select open_dataset method based on current engine."""
        if isinstance(engine, str):
            engines = list_engines()
            if engine not in engines:
>               raise ValueError(
                    f"unrecognized engine {engine} must be one of: {list(engines)}"
                )
E               ValueError: unrecognized engine scipy must be one of: ['netcdf4', 'h5netcdf', 'store']

/mnt/e/Git/xarray/xarray/backends/plugins.py:156: ValueError
================================================= short test summary info =================================================
FAILED xarray/tests/test_backends.py::TestH5NetCDFFileObject::test_open_fileobj - ValueError: unrecognized engine scipy ...
=========================== 1 failed, 458 passed, 818 skipped, 18 xfailed, 6 xpassed in 25.08s ===========================

[keewis]

Good catch, @joeperdefloep, this should be fixed by decorating the test with requires_scipy. Could you make that change?

[github-actions]

Unit Test Results

         6 files           6 suites   52m 19s ⏱️
16 163 tests 14 448 ✔️ 1 715 💤 0 ❌
90 174 runs  82 061 ✔️ 8 113 💤 0 ❌

Results for commit fab8263.

♻️ This comment has been updated with latest results.

[dcherian]

Looks great. Thanks @joeperdefloep . I see this is your first contribution to xarray. Welcome!