Issue #23014: Make importlib.abc.Loader.create_module() required when

Issue #23014: Make importlib.abc.Loader.create_module() required when
importlib.abc.Loader.exec_module() is also defined.

Before this change, create_module() was optional **and** could return
None to trigger default semantics. This change now reduces the
options for choosing default semantics to one and in the most
backporting-friendly way (define create_module() to return None).