MNT: move matplotlibrc.template to mpl-data - #19602

PR Summary

Alternative to #19588, closes #19578

This simply makes lib/matplotlib/mpl-data/matplotlibrc the file under source control and soft-links back to matplotlibrc.template. setup.py now modifies the file in place if the backend needs to be edited.

More detail:

Previously matplotlibrc.template was changed in #15029 to be the canonical place to get the defaults for all the rcParams. However, these were read during __init__.py from lib/matplotlib/mpl-data/matplotlibrc. lib/matplotlib/mpl-data/matplotlibrc doesn't exist in the repo, and only gets created when setup.py is called. We can't read the defaults from matplotlbrc.template because that file probably doesn't exist in most user installs.

Aside from being somewhat rickety, the problem is that developers working from an editable install don't want to always call setup.py as it requires compiling the C extensions, and hence their lib/matplotlib/mpl-data/matplotlibrc can get out of sync with the library.

The solution here is to simply make lib/matplotlib/mpl-data/matplotlibrc the canonical source of the defaults, add it to version control, and softlink it to matplotlib.template. setup.py can change lib/matplotlib/mpl-data/matplotlibrc in-place (I think)

See also:

• Backend related issues with matplotlib 3.0.0rc1 #11844
• Get default params from matplotlibrc.template. #14929
• Get default params from matplotlibrc.template. #15029

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• 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).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).