PR Summary

Right now we are deprecating access to the global colormaps, but users can only avoid it by doing newcmap = copy.copy(cmap) which is a bit heavy-handed. Here we provide a simple copy method to Colormap so we can do newcmap = cmap.copy().

See discussion in #19609, #16991, #18503

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).