[3.9] bpo-42567: [Enum] call __init_subclass__ after members are added (GH-23714...

[3.9] bpo-42567: [Enum] call __init_subclass__ after members are added (GH-23714) (GH-23772)

When creating an Enum, `type.__new__` calls `__init_subclass__`, but at that point the members have not been added.

This patch suppresses the initial call, then manually calls the ancestor `__init_subclass__` before returning the new Enum class.
(cherry picked from commit 6bd94de168b58ac9358277ed6f200490ab26c174)