Bug: Allow np.timedelta64 objects to index TimedeltaIndex
- [x] closes #20393
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

`is_integer(label)` would catch `np.timedelta64` objects due to https://github.com/numpy/numpy/issues/10685 (I believe), so ensure that the label is also not a timedelta64 dtype