Make stubtest handle `__new__` in Enums

[hauntsaninja]

See https://github.com/python/typeshed/actions/runs/7606392260/job/20712113127?pr=11299 / python/typeshed#11299 / python/typing-council#11

stubtest doesn't handle this case correctly. The logic I had is simplistic:

mypy/mypy/stubtest.py

Line 1092 in 1fd29ac

if isinstance(runtime, enum.Enum):