We noticed a behavior change in Python3.10.0a4 (it works properly in Python3.10.0a3) when running Django's test suite[1]. Subclassing `enum.Enum` with `ipaddress.IPv4Network/IPv6Network` raises `TypeError`, e.g.

>>> import enum
>>> import ipaddress
>>> class NewEnum(ipaddress.IPv4Address, enum.Enum):
...     pass
... 
>>> class NewEnum(ipaddress.IPv4Network, enum.Enum):
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.10/enum.py", line 239, in __new__
    enum_class.__bases__ = enum_class.__bases__[1:] #or (object, )
  File "/usr/local/lib/python3.10/enum.py", line 483, in __setattr__
    super().__setattr__(name, value)
TypeError: __bases__ assignment: 'IPv4Network' object layout differs from '_NoInitSubclass'

It still works for `ipaddress.IPv4Address/IPv6Address`.

[1] https://github.com/django/django/blob/102d92fc09849e1a9004dd3f9a14a0ea9ca392cd/tests/model_enums/tests.py#L225-L235

That would be due to some changes to try and get `Enum` and `__init_subclass__` to work together.

I'll revert those changes.  Thank you for the report.

New changeset a581a868d97f649aedf868a1d27865a10925c73a by Ethan Furman in branch 'master':
bpo-42851: [Enum] remove brittle __init_subclass__ support (GH-24154)
https://github.com/python/cpython/commit/a581a868d97f649aedf868a1d27865a10925c73a

New changeset 9ab4dd452287169f08a8cf4d4c68c2139f8de714 by Ethan Furman in branch '3.9':
[3.9] bpo-42851: [Enum] remove brittle __init_subclass__ support (GH-24154) (GH-24155)
https://github.com/python/cpython/commit/9ab4dd452287169f08a8cf4d4c68c2139f8de714

Ethan, Thanks for the patch.