Fix type hint of `key` on `Visitable.__class_getitem__()`

Fix type hint of `key` on `Visitable.__class_getitem__()`

Fixed the type annotation for ``__class_getitem__()`` as applied to the
``Visitable`` class at the base of expression constructs to accept ``Any``
for a key, rather than ``str``, which helps with some IDEs such as PyCharm
when attempting to write typing annotations for SQL constructs which
include generic selectors.  Pull request courtesy Jordan Macdonald.

Fixes: #9878
Closes: #9881
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9881
Pull-request-sha: 5aefaaf1065d1f946137a8b2f5be0767856cf9b0

Change-Id: I956e6d41142c50d870a142c3e99567a7ed2f7114