Fix the docstring of _get_textbox. (Text.get_extents doesn't exist.)

Inline _draw_bbox into its sole call site (the first half is the same
as update_bbox_position_size; this does mean calling
convert_xunits/convert_yunits twice but that should be cheap compared
to everything else).

Annotation.draw doesn't need to also call update_bbox_position_size
as Text.draw will do it (I think the call was duplicated because the
draw-a-bbox functionality was present in Annotation before it also moved
to Text).

rcParams -> mpl.rcParams.

Remove an extraneous import.

PR Summary

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 pydocstyle<4 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).