The deleted comment was really only relevant to Py2.
Preserving _cid_gen (which is picklable) is necessary to later enable
careful pickling of select callbacks, such as 3d mouse handlers (#10843)
(otherwise, the same cid may be assigned twice, one before pickling and
once after unpickling).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code isFlake 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