prevent asynccontextmanager and sync contextmanager drifting

[graingert]

I think by adopting the testing approach taken by AsyncExitStack and ExitStack for contextlib.contextmanager it should be possible to prevent contextlib.asynccontextmanager from drifting in implementation

class TestAsyncExitStack(TestBaseExitStack, unittest.TestCase):
    class SyncAsyncExitStack(AsyncExitStack):

[graingert]

I think doing this at the same time makes sense too #95888

Should make the tests run a bit faster and the traceback hacking clearer