Keeping around lastevent can delay garbage collection of a torn-down axes, and can also keep around an actual GUI event object (guiEvent) associated with a long-torn down widget, which can be problematic for some GUI toolkits.

Instead, only keep track of the last inaxes attribute.

Unfortunately there are no tests for axes_enter_event/axes_leave_event, but this can be manually tested by running the
event_handling/figure_axes_enter_leave.py example.

See #24820, #22211 (comment).

PR Summary

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst