Fix typing generics in PostgreSQL range types.

Fix typing generics in PostgreSQL range types.

Correctly type PostgreSQL RANGE and MULTIRANGE types as ``Range[T]``
and ``Sequence[Range[T]]``.
Introduced utility sequence ``MultiRange`` to allow better
interoperability of MULTIRANGE types.

Fixes #9736
Closes: #10625
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10625
Pull-request-sha: 2c17bc5f922a2bdb805a29e458184076ccc08055

Change-Id: I4f91d0233b29fd8101e67bdd4cd0aa2524ab788a