Note that the previously documented "Call signature" did not mention "transparent".

Also adjust tools/boilerplate.py so that pyplot.savefig can also benefit from this change; this implies generalizing
axes_cmappable_method_template to support general postambles, also for Figure methods.

See the new signature of pyplot.savefig for the end result.

Edit: This currently fails test_agg.py::test_webp_alpha because facecolor=None is now explicitly passed to Figure.savefig, which preventskwargs.setdefault("facecolor", "none") from taking effect, andfacecolor=None is instead interpreted as "use the rcparam" :/ Probably easy to fix, but needs some thought about strict backcompat.

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (installflake8-docstrings and runflake8 --docstring-convention=all).

Documentation

• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a.. versionadded:: directive in the docstring and documented indoc/users/next_whats_new/
• API changes are marked with a.. versionchanged:: directive in the docstring and documented indoc/api/next_api_changes/
• Release notes conform with instructions innext_whats_new/README.rst ornext_api_changes/README.rst