BUG: Compat for pre-0.20 TimedeltaIndex and Float64Index pickles #19939
- [x] closes #19939 
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Added pre-0.20 locations of TimedeltaIndex and Float64Index to pickle_compat.py.