[3.11] Improve test coverage for is_typeddict (GH-104884) (#104888)

In particular, it's important to test that is_typeddict(TypedDict)
returns False.
(cherry picked from commit 1497607a8e99f1103c40368dd5f9057f0146a520)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>