This makes the np.timedelta64 and np.datetime64 scalar types generic with one type parameter, matching the return type of their .item() (and .tolist()) methods.
The type parameters are bound to datetime.timedelta | int | None and datetime.date | datetime.datetime | int | None for timedelta64 and datetime64, respectively.
Both type-parameters are optional for backwards compatibility, and relate to the time-units in the following way:

Static type checkers are aware of these units, and will infer e.g. timedelta64(1, "D") as timedelta64[timedelta], timedelta64("NaT") as timedelta64[None], and datetime64(b"today") as datetime64[date].

The (scalar) arithmetic operators have been made aware of the type-parameters. So NaT will always propagate when adding, subtracting, etc, and the addition of datetime64[date] and timedelta64[int] will be inferred as datetime64[int].

Note that this only applies to the scalar operators. So the ndarray operators, for instance, do not yet support these generic timedelta64 and datetime64 type parameters, and will continue to behave as if these type parameters didn't exist.