REGR: Series.loc with a MultiIndex containing Timestamp raises InvalidIndexError
ci testing fix in https://github.com/pandas-dev/pandas/issues/35858#issuecomment-678683559

- [ ] closes #35858
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
