There's already a better check from the minversion entry in pytest.ini,
which will actually error out cleanly if trying to run the tests with a
too-old pytest.

(For example the setupext check did not prevent #12945 (comment), whereas the minversion in pytest.ini would have done so (#12975).)

Also that's one fewer place to update when bumping the pytest minversion.

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