Add DateTime to typing for diff()

[cp2boston]

• [ x] I have searched the issues of this repo and believe that this is not a duplicate.
• [ x] I have searched the documentation and believe that my question is not covered.

Feature Request

This doesn't affect operations, but it does cause my linter to flag. The diff method:

    def diff(  # type: ignore[override]
        self, dt: datetime.datetime | None = None, abs: bool = True
    ) -> Interval:
        """
        Returns the difference between two DateTime objects represented as an Interval.
        """
        if dt is None:
            dt = self.now(self.tz)

        return Interval(self, dt, absolute=abs)

will complain if the dt type is a Pendulum DateTime object.