Improve operator methods for dateutil.relativedelta stubs:
* `__add__` operator method could return other types than `relativedelta` (`datetime.date` or `datetime.datetime`)
* use specific types of operators args instead of Any
* mypy currently does not handle `Union` in op methods (see python/mypy#2129, python/mypy#1442, python/mypy#1264 for details), so I've overloaded it directly
