PR Summary

Made some suggested changes related to issue #20698.

• Merged Collection._uniform_offsets and Collection._offsets, removed the former, kept the latter. Any function dependent on this variable was removed.
• Removed Collection._offsetsNone, replaced with a check if offsets were non-zero where used.
• Added Collection.set_offset_transform().

PR Checklist

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

I did add Collection.set_offset_transform(), but this isn't really a new feature, since it can already be set during initialization, so I'm not sure if/how it should be documented. I can add a mention if necessary.

Update 2021-07-23 09:47:

• Copied the body of Artist.set_transform() to Collection.set_offset_transform() to be consistent.
• Added a release note.