Register flaky pytest marker.

[anntzer]

Otherwise, running the test suite without installing
pytest-rerunfailures now results in

_____________________________ ERROR collecting lib/matplotlib/tests/test_backend_ps.py _____________________________
lib/matplotlib/tests/test_backend_ps.py:29: in <module>
    @pytest.mark.flaky(reruns=3)
../../../.local/lib/python3.7/site-packages/_pytest/mark/structures.py:324: in __getattr__
    PytestUnknownMarkWarning,
E   _pytest.warning_types.PytestUnknownMarkWarning: Unknown pytest.mark.flaky - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
collected 0 items / 1 errors

(The alternative is to add a hard requirement on pytest-rerunfailures to
run the test suite, but that seems overkill.)

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