PR Summary

In Python 3.10 the repr and str representation of Enums changed from

str: 'ClassName.NAME' -> 'NAME'
repr: '<ClassName.NAME: value>' -> 'ClassName.NAME'

which is more consistent with what str/repr should do, however this breaks
boilerplate which needs to get the ClassName.NAME version in all versions of
Python. Thus, we locally monkey patch our preferred str representation in
here.

bpo-40066
python/cpython#22392

Other options considered:

• patch MouseButton.__str__ in matplotlib.backend_bases. This has the advantage of maintaining stability of str(MouseButton.Left) over time, but this feels like something we should not fight upstream on. If they chose to break this intentionally we should follow them and not make our Enum "special"
• re-writing

  matplotlib/tools/boilerplate.py

  Lines 122 to 125 in f6790c0

  	signature = str(signature.replace(parameters=[
  	param.replace(default=value_formatter(param.default))
  	if param.default is not param.empty else param
  	for param in params]))

  to special case MouseButton, but that would more-or-less involve vendoring the str methods from both inspect.Signature and inspect.Parameter which are about a screen of code in total.

PR Checklist

• Has pytest style unit tests (and pytest passes). (this fixes a failure on py310)
• Is Flake 8 compliant (run flake8 on changed files to check).
• [N/A] New features are documented, with examples if plot related.
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).