For valid types which encapsulate other types, like typing.List or typing.Tuple, you can declare what's contained in them like so:

foo: typing.List[int] = []

Unfortunately weakref.ReferenceType does not allow you to do this.  You get the error:
>>> foo: weakref.ReferenceType[typing.Any] = None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'type' object is not subscriptable

so either ReferenceType should be fixed or a new type made available.