REF/TYP: arrays.datetimelike
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@simonjayhawkins this is still giving me two mypy complaints locally:

```
pandas/core/arrays/datetimelike.py:549: error: Too many arguments for "object"  [call-arg]
pandas/core/arrays/datetimelike.py:556: error: Too many arguments for "object"  [call-arg]
```

These lines are both calls to `self._scalar_type(foo)`

_scalar_type is annotated as `Type[DatetimeLikeScalar]`, which is a TypeVar for Period/Timestamp/Timedelta.  Is there a non-ignoring solution here?