Make rcParams.copy() return a new RcParams instance.

[anntzer]

... so that rcParams.update(<previously-copied-rcparams-instance>)
doesn't emit DeprecationWarnings even if some entries are deprecated.
(See comment in tests.)

Supersedes #21015.

Closes #13118. Closes #15781. Closes #20249.

PR Summary

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).

[jklymak]

This looks fine to me, though I'm not 100% up on all our rcParam semantics.

[tacaswell]

This PR is affected by a re-writing of our history to remove a large number of accidentally committed files see discourse for details.

To recover this PR it will need be rebased onto the new default branch (main). There are several ways to accomplish this, but we recommend (assuming that you call the matplotlib/matplotlib remote "upstream"

git remote update
git checkout main
git merge --ff-only upstream/main
git checkout YOUR_BRANCH
git rebase --onto=main upstream/old_master
# git rebase -i main # if you prefer
git push --force-with-lease   # assuming you are tracking your branch

If you do not feel comfortable doing this or need any help please reach out to any of the Matplotlib developers. We can either help you with the process or do it for you.

Thank you for your contributions to Matplotlib and sorry for the inconvenience.