restore old *args approach for MutableDict.pop()

restore old *args approach for MutableDict.pop()

The typing change in ba0e508141206efc55cdab91df21c1
changed the semantics of pop() and possibly setdefault() in order to
try working at runtime with a two-argument signature.  however
the implementation for this in cpython likely uses a strict `*args`
approach where the lack of the second parameter is explicit, rather
than matching to a constant.  Restore the old implementation inside
of a "not TYPE_CHECKING" block while keeping the type annotated forms
intact for typing only.

Fixed regression caused by typing added to ``sqlalchemy.ext.mutable`` for
:ticket:`8667`, where the semantics of the ``.pop()`` method changed such
that the method was non-working.  Pull request courtesy Nils Philippsen.

Fixes: #9380
Closes: #9381
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9381
Pull-request-sha: fd903ce1b949d2af26ceb6c2159ad84aab007f3d

Change-Id: I213e52f51a795801aacf05307e38cc8c89b54e12