See changelog note.

There are also other advantages with contextmanager, e.g. reusing a
context multiple times works

ctx = rc_context(...)
with ctx: ... # in the context
... # out of the context
with ctx: ... # in the context again

(with a test for the decorator form, which is equivalent) but in
practice this will often run into the issue of early/late resolution of
rcParams, so I didn't mention it in the changelog.

xkcd() still needs to be implemented manually in terms of
__enter__/__exit__ but at least we know that creation of the
contextmanager cannot fail, so things are simpler.

Also we don't need to check for whether the backend has been
resolved because rcParams now just prevent re-setting the backend to
auto_backend_sentinel.
(https://github.com/matplotlib/matplotlib/pull/11896/files#diff-9ffb0d1db51a67ee4f37528e00715b3cR852)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way