Groundwork for task 3, plotting with function(x,y), test file for task 3, bug fixes

PR Summary

This PR lays the groundwork for Task 3 issue matplotlib#23378. It allows users to call ax.plot() with the argument "hover=usr_func", where usr_func is a user-defined function of x,y that returns modified x and y coordinates. These modified x and y coordinates are then displayed in the tooltip in the bottom right hand corner of the screen. This PR also includes a test file for this feature from the perspective of a user, and it also includes a bug fix for a test file that was triggering an error when it shouldn't have been; this code was removed from the codebase.

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