Get rcParams from mpl

[oscargus]

PR Summary

Realized that this was quite inconsistent. It is still not really consistent, but I doubted half-ways that there is a preferred way...

Related to #23680 (almost closes it...)

So, is there are preferred way among

import matplotlib as mpl
...
mp.rcParams[...]

and

from matplotlib import rcParams
...
rcParams[...]

?

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).

[timhoffm]

Use mpl.rcParams. I've written down the policy in #23680.

[oscargus]

OK! Then there are a few more files to change.

[oscargus]

I have changed "all" instances. In pyplot it is still imported. Some tests, use plt.rcParams, which seems to be on purpose and some should probably be changed to use rcContext.

But with respect to this PR I am happy for now.