CLN: Add type hinting to base classes (#28926)
- [x] partially addresses #28926
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [N/A] whatsnew entry

Linked/Duplicate PR #28979 Ah, I'm sorry @aaditya-panik, thought I checked everything that someone was already working on... No worries @jjlkant, just added the comment to link the two PRs so that it's easy for the maintainers to discuss if needed 😄  Will close this pull request, probably will be implemented using the continuation from https://github.com/pandas-dev/pandas/pull/28979#pullrequestreview-301909855