matplotlib 3.5.0rc1 raises AttributeError in test_no_numpy_warnings

[dhomeier]

Description

Uncaught exception in visualization.wcsaxes test because matplotlib 3.5.0rc does not identify as MATPLOTLIB_DEV.

Expected behavior

Intercept exception as required by mpl version.

Actual behavior

Failure in

_________________________________________________________________________ test_no_numpy_warnings[contours] _________________________________________________________________________
ignore_matplotlibrc = None, tmpdir = local('/private/var/folders/bb/f_z2vsjd21d_p0jpqxzhhwhh00027w/T/pytest-of-derek/pytest-87/test_no_numpy_warnings_contour0')
grid_type = 'contours'

    @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)
    
        if MATPLOTLIB_DEV and grid_type == 'contours':
            ctx = pytest.raises(AttributeError, match='dpi')
        else:
            ctx = nullcontext()
    
        with pytest.warns(None) as warning_lines, ctx:
>           plt.savefig(tmpdir.join('test.png').strpath)

/opt/lib/python3.10/site-packages/astropy/visualization/wcsaxes/tests/test_misc.py:90: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/opt/lib/python3.10/site-packages/matplotlib/pyplot.py:959: in savefig
    fig.canvas.draw_idle()   # need this if 'transparent=True' to reset colors
/opt/lib/python3.10/site-packages/matplotlib/backend_bases.py:2050: in draw_idle
    self.draw(*args, **kwargs)
/opt/lib/python3.10/site-packages/matplotlib/backends/backend_agg.py:434: in draw
    self.figure.draw(self.renderer)
/opt/lib/python3.10/site-packages/matplotlib/artist.py:73: in draw_wrapper
    result = draw(artist, renderer, *args, **kwargs)
/opt/lib/python3.10/site-packages/matplotlib/artist.py:50: in draw_wrapper
    return draw(artist, renderer)
/opt/lib/python3.10/site-packages/matplotlib/figure.py:2803: in draw
    mimage._draw_list_compositing_images(
/opt/lib/python3.10/site-packages/matplotlib/image.py:132: in _draw_list_compositing_images
    a.draw(renderer)
/opt/lib/python3.10/site-packages/astropy/visualization/wcsaxes/core.py:456: in draw
    super().draw(renderer, **kwargs)
/opt/lib/python3.10/site-packages/matplotlib/artist.py:50: in draw_wrapper
    return draw(artist, renderer)
/opt/lib/python3.10/site-packages/matplotlib/axes/_base.py:3082: in draw
    mimage._draw_list_compositing_images(
/opt/lib/python3.10/site-packages/matplotlib/image.py:132: in _draw_list_compositing_images
    a.draw(renderer)
/opt/lib/python3.10/site-packages/matplotlib/artist.py:50: in draw_wrapper
    return draw(artist, renderer)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <matplotlib.collections.PathCollection object at 0x120ff9de0>, renderer = <matplotlib.backends.backend_agg.RendererAgg object at 0x120b68cd0>

    @artist.allow_rasterization
    def draw(self, renderer):
>       self.set_sizes(self._sizes, self.figure.dpi)
E       AttributeError: 'NoneType' object has no attribute 'dpi'

/opt/lib/python3.10/site-packages/matplotlib/collections.py:976: AttributeError
================================================================================= warnings summary =================================================================================
../../opt/lib/python3.10/site-packages/_pytest/cacheprovider.py:428
  /opt/lib/python3.10/site-packages/_pytest/cacheprovider.py:428: PytestCacheWarning: could not create cache path /opt/lib/python3.10/site-packages/astropy/visualization/wcsaxes/.pytest_cache/v/cache/nodeids
    config.cache.set("cache/nodeids", sorted(self.cached_nodeids))

../../opt/lib/python3.10/site-packages/_pytest/cacheprovider.py:382
  /opt/lib/python3.10/site-packages/_pytest/cacheprovider.py:382: PytestCacheWarning: could not create cache path /opt/lib/python3.10/site-packages/astropy/visualization/wcsaxes/.pytest_cache/v/cache/lastfailed
    config.cache.set("cache/lastfailed", self.lastfailed)

../../opt/lib/python3.10/site-packages/_pytest/stepwise.py:49
  /opt/lib/python3.10/site-packages/_pytest/stepwise.py:49: PytestCacheWarning: could not create cache path /opt/lib/python3.10/site-packages/astropy/visualization/wcsaxes/.pytest_cache/v/cache/stepwise
    session.config.cache.set(STEPWISE_CACHE_DIR, [])

-- Docs: https://docs.pytest.org/en/stable/warnings.html
============================================================================= short test summary info ==============================================================================
FAILED ../../opt/lib/python3.10/site-packages/astropy/visualization/wcsaxes/tests/test_misc.py::test_no_numpy_warnings[contours] - AttributeError: 'NoneType' object has no attri...
============================================================== 1 failed, 184 passed, 45 skipped, 3 warnings in 10.72s ==============================================================

Steps to Reproduce

With astropy 5.0rc1, matplotlib 3.5.0rc1

>>> import astropy
>>> import matplotlib
>>> from packaging.version import Version
>>> Version(matplotlib.__version__)
<Version('3.5.0rc1')>
>>> Version(matplotlib.__version__).is_devrelease
False
>>> astropy.test(package='visualization.wcsaxes')

System Details

[same for Python 3.9.6, 3.9.8]
Running tests with Astropy version 5.0rc1.
Running tests in /opt/lib/python3.10/site-packages/astropy/visualization/wcsaxes.
Platform: macOS-10.14.6-x86_64-i386-64bit
Executable: /opt/bin/python3.10
Full Python Version:
3.10.0 (default, Oct 5 2021, 03:51:16) [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: 1.21.2
Scipy: 1.7.1
Matplotlib: 3.5.0rc1
h5py: 3.5.0
Pandas: 1.3.3
PyERFA: 2.0.0
Cython: 0.29.24
Scikit-image: not available
asdf: 2.8.1

[astrofrog]

I think we should fix the underlying issue here which is that savefig should not be erroring in the first place with any version.

[astrofrog]

I've managed to narrow it down to the code in this issue: matplotlib/matplotlib#21569 - this is related to a change in contours from LineCollection to PathCollection, and the fact the contour lines aren't removed correctly.