Fix mypy in pandas/tests/arithmetic/test_datetime64.py
GH28926, mypy did not like DatetimeIndex's __new__,
and couldn't find a method

- [x] addresses part of #28926
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
