issues: 346823063

Some of our tests for warnings currently fail when run using an old version of pytest. The problem appears to be that we rely on pytest.warns() accepting subclasses rather exact matches.

This was fixed upstream in pytest (https://github.com/pytest-dev/pytest/pull/2166), but we still should specify the more specific warning types in xarray.

``` =================================== FAILURES =================================== __ TestEncodeCFVariable.testmissing_fillvalue ____

self = <xarray.tests.test_conventions.TestEncodeCFVariable testMethod=test_missing_fillvalue>

def test_missing_fillvalue(self):
    v = Variable(['x'], np.array([np.nan, 1, 2, 3]))
    v.encoding = {'dtype': 'int16'}
    with pytest.warns(Warning, match='floating point data as an integer'):

      conventions.encode_cf_variable(v)

E Failed: DID NOT WARN

tests/test_conventions.py:89: Failed

_____ TestAlias.test _____

self = <xarray.tests.test_utils.TestAlias testMethod=test>

def test(self):
    def new_method():
        pass
    old_method = utils.alias(new_method, 'old_method')
    assert 'deprecated' in old_method.__doc__
    with pytest.warns(Warning, match='deprecated'):

      old_method()

E Failed: DID NOT WARN

tests/test_utils.py:28: Failed

___ TestIndexVariable.test_coordinate_alias ______

self = <xarray.tests.test_variable.TestIndexVariable testMethod=test_coordinate_alias>

def test_coordinate_alias(self):
    with pytest.warns(Warning, match='deprecated'):

      x = Coordinate('x', [1, 2, 3])

E Failed: DID NOT WARN

tests/test_variable.py:1752: Failed ____ TestAccessor.test_register ______

self = <xarray.tests.test_extensions.TestAccessor testMethod=test_register>

def test_register(self):

    @xr.register_dataset_accessor('demo')
    @xr.register_dataarray_accessor('demo')
    class DemoAccessor(object):
        """Demo accessor."""

        def __init__(self, xarray_obj):
            self._obj = xarray_obj

        @property
        def foo(self):
            return 'bar'

    ds = xr.Dataset()
    assert ds.demo.foo == 'bar'

    da = xr.DataArray(0)
    assert da.demo.foo == 'bar'

    # accessor is cached
    assert ds.demo is ds.demo

    # check descriptor
    assert ds.demo.__doc__ == "Demo accessor."
    assert xr.Dataset.demo.__doc__ == "Demo accessor."
    assert isinstance(ds.demo, DemoAccessor)
    assert xr.Dataset.demo is DemoAccessor

    # ensure we can remove it
    del xr.Dataset.demo
    assert not hasattr(xr.Dataset, 'demo')

    with pytest.warns(Warning, match='overriding a preexisting attribute'):
        @xr.register_dataarray_accessor('demo')

      class Foo(object):

E Failed: DID NOT WARN

tests/test_extensions.py:60: Failed

```