TYP: timedeltas.pyi
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

@simonjayhawkins mypy is unhappy about `Timedelta.__new__` potentially returning `NaT`.  Thoughts on how to handle this?  (it'll come up again in Timestamp and Period)

```
pandas/_libs/tslibs/timedeltas.pyi:51: error: "__new__" must return a class instance (got "Union[Timedelta, NaTType]")  [misc]
```