Since mypy doesn't accept redefinition of the same variable within the
same scope, we need to use type annotations with Union to correctly
type aggregate_table. Note that the type cast is necessary for mypy to
narrow the type of aggregate_table.
For types like `Union[Realm, UserProfile, Stream]` and
`Union[AnonymousUser, AbstractBaseUser]`, we need assertions to
tell mypy which type we would be expecting.
Tim Abbott
PIG208