PR Summary

mostly performed by running pycln, but then manually audited

After it was brought up in #25728 (comment), I did a bit of a review, and consulted the python/typeshed repo for guidance on best practices for type stub files.
That repo has a precommit hook to run pycln -a on all files, which removes any imports that are implementation details that are not used in the public interface of the module.

Since that is the prevailing best practice of the relative authority on the matter, I decided to run pycln over our code.

The following additional changes were made after auditing the results:

• The <...>_COLORS dictionaries are explicitly exported in the colors module
  • that they are imported is an implementation detail, but it is public within this module, as far as I can tell
• TextPath is exported from the text module.
  • despite existing in the textpath module which is at least arguably public (at least by way of not having a prepended _)
  • TextPath is documented in text and the textpath module is undocumented
• A few others I questioned, but decided were not intended to be public api:
  • Helper functions in font_manager
  • legend_handler in the legend module (legend_handler is its own public module, should be imported directly)
  • Helper functions for layout_manager (even the ones that are documented contain a warning that they are considered private)
• mathtext reexports values from _mathtext
• A few import foo as foo are removed as those are holdout from generating the stub files and are not intended to be exported
• All changes to .py files were excluded, though perhaps some should be removed
  • Files that would have changes:
    • galleries/examples/statistics/time_series_histogram.py
    • galleries/examples/ticks/date_formatters_locators.py
    • lib/matplotlib/init.py
    • lib/matplotlib/axes/_axes.py
    • lib/matplotlib/backends/backend_template.py
    • lib/matplotlib/backends/qt_compat.py
    • lib/matplotlib/pylab.py
    • lib/matplotlib/pyplot.py
    • lib/matplotlib/tests/test_determinism.py
  • many of these are actually intended to be exported, cause side effects, or are used via eval (in galleries)
  • Unused imports are generally caught by flake8 ci, but have several instances of per-file-ignores for that error

pycln could be added as a pre-commit hook here if we were interested, but would need to be properly configured, and current personal leaning is that it is not a big enough boon to do so, though checking when the topic comes up like it did here may be warranted.

PR Checklist

Linked Issue

• [N/A] Added "closes #0000" in the PR description to link it to the original issue.

Documentation and Tests

• [N/A] Has pytest style unit tests (and pytest passes)
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] New plotting related features are documented with examples.

Release Notes

• [N/A] New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• [N/A] API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• [N/A] Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst