PERF: load plotting entrypoint only when necessary
- [x] closes #41492
- [x] tests added / passed (I only ran plotting/test_backend.py, which seemed appropriate)
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry