PY_SSIZE_T_CLEAN warning with Python 3.9 and Matplotlib 3.0

Description

With matplotlib 3.0.x test_no_numpy_warnings runs into an unexpected deprecation warning.

Steps to Reproduce

astropy.test(package='visualization', args='-vk numpy_warn')
...
Running tests with Astropy version 4.2.dev1150+g3421137b6.d20201027.
Running tests in lib/python3.9/site-packages/astropy/visualization.

Date: 2020-10-27T13:58:40

Platform: macOS-10.14.6-x86_64-i386-64bit

Executable: /opt/bin/python3.9

Full Python Version: 
3.9.0 (default, Oct 10 2020, 10:27:03) 
[Clang 11.0.0 (clang-1100.0.33.17)]

encodings: sys: utf-8, locale: UTF-8, filesystem: utf-8
byteorder: little
float info: dig: 15, mant_dig: 15

Package versions: 
Numpy: 1.19.2
Scipy: 1.5.3
Matplotlib: 3.0.3
h5py: not available
Pandas: not available
PyERFA: 1.7.0
Cython: 0.29.21
Scikit-image: not available
asdf: 2.7.1

Using Astropy options: remote_data: any.

hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/Users/derek/.hypothesis/examples')
rootdir: /Users/derek
plugins: arraydiff-0.3, xdist-2.1.0, astropy-header-0.1.2, hypothesis-5.16.0, cov-2.10.1, flaky-3.7.0, asdf-2.7.1, remotedata-0.3.2, mock-3.3.1, forked-1.3.0, openfiles-0.5.0, doctestplus-0.8.0, expect-1.1.0, flake8-1.0.6
collected 479 items / 477 deselected / 2 selected                                                                                                                                  

lib/python3.9/site-packages/astropy/visualization/wcsaxes/tests/test_misc.py::test_no_numpy_warnings[lines] FAILED                                                           [ 50%]
lib/python3.9/site-packages/astropy/visualization/wcsaxes/tests/test_misc.py::test_no_numpy_warnings[contours] FAILED                                                        [100%]

===================================================================================== FAILURES =====================================================================================
__________________________________________________________________________ test_no_numpy_warnings[lines] ___________________________________________________________________________

ignore_matplotlibrc = None, tmpdir = local('/private/var/folders/7l/j6rbh6fw8xjc4w0059bv63_h00027w/T/pytest-of-derek/pytest-18/test_no_numpy_warnings_lines_0'), grid_type = 'lines'

    @pytest.mark.parametrize('grid_type', ['lines', 'contours'])
    def test_no_numpy_warnings(ignore_matplotlibrc, tmpdir, grid_type):
        ax = plt.subplot(1, 1, 1, projection=WCS(TARGET_HEADER))
        ax.imshow(np.zeros((100, 200)))
        ax.coords.grid(color='white', grid_type=grid_type)
    
        with pytest.warns(None) as warning_lines:
            plt.savefig(tmpdir.join('test.png').strpath)
    
        # There should be no warnings raised if some pixels are outside WCS
        # (since this is normal).
        # BUT catch_warning was ignoring some warnings before, so now we
        # have to catch it. Otherwise, the pytest filterwarnings=error
        # setting in setup.cfg will fail this test.
        # There are actually multiple warnings but they are all similar.
        for w in warning_lines:
            print(w.message)
            w_msg = str(w.message)
>           assert ('converting a masked element to nan' in w_msg or
                    'No contour levels were found within the data range' in w_msg or
                    'np.asscalar(a) is deprecated since NumPy v1.16' in w_msg)
E           assert ('converting a masked element to nan' in "PY_SSIZE_T_CLEAN will be required for '#' formats" or 'No contour levels were found within the data range' in "PY_SSIZE_T_CLEAN will be required for '#' formats" or 'np.asscalar(a) is deprecated since NumPy v1.16' in "PY_SSIZE_T_CLEAN will be required for '#' formats")

lib/python3.9/site-packages/astropy/visualization/wcsaxes/tests/test_misc.py:95: AssertionError
------------------------------------------------------------------------------- Captured stdout call -------------------------------------------------------------------------------
PY_SSIZE_T_CLEAN will be required for '#' formats
_________________________________________________________________________ test_no_numpy_warnings[contours] _________________________________________________________________________

...

This is related to https://bugs.python.org/issue36381 and seems fixed as of Matplotlib 3.1. Adding it to the list of expected warnings would be trivial, but if the minimum version is increased to >= 3.1.0 as per https://github.com/astropy/astropy/pull/10900#issuecomment-717211293 there will probably be no further action needed.

[pllim]

Seeing a notification about this now reminded me of

https://github.com/astropy/astropy/blob/0dabeb4bb1bb5b66bc7b93f632c77ca4ad0ba413/setup.cfg#L126

[pllim]

How about #10956?