Fix test_imshow_pil on Windows.

[anntzer]

test_imshow_pil currently fails on Windows, but this was not detected
because PIL is not installed on the Appveyor CI so the test is skipped.

Noted while working on #13077.
The Pillow issue is python-pillow/Pillow#3823.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[jkseppan]

Is there a reason that we don't install PIL on Appveyor, or could it be added in .appveyor.yml?

[anntzer]

I think originally because the appveyor tests were extremely slow and a real bottleneck.
OTOH I guess it is nice to have at least one test instance without Pillow to show that Matplotlib indeed works fine in its absence.

[jkseppan]

Right, I remember that the Appveyor tests were really slow at some time. Obviously we can’t test every combination of dependencies.