Fix passing Path to ps backend when text.usetex rc is True.

[anntzer]

... and misc. cleanups to test_backend_ps (more pathlib + remove an
unnecessary rc_context() as tests always reset the rcs at exit).

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

[WeatherGod]

How does that one particular test get its context reset automatically? It has no decorators.

[anntzer]

with the autouse fixture at https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/testing/conftest.py#L17.

note, for example, that the test immediately above (test_savefig_to_stringio) also modifies the rcparams but does not bother to restore them.