Performance of maybe_box_datetimelike #30520
- [x] closes #30520
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] `maybe_box_datetimelike` avoids unnecessary conversion of `value` to `tslibs.Timestamp` or `tslibs.Timedelta` if `value` is already an instance of `tslibs.Timestamp` or `tslibs.Timedelta`.

Hello @ivan-vasilev! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-01-01 23:40:02 UTC See my comment in the issue: https://github.com/pandas-dev/pandas/issues/30520#issuecomment-569521056.  Is this really the appropriate place for the performance fix?  Or should this be done in the `Timestamp` constructor? linting issue should be fixable by running `black pandas/core/common.py` this is subsumed by #30676 (though if you still think there is a perf benefit after that certainly ping).