[Enum] move member creation to __set_name__ in order to support __init_subclass__

[ethanfurman]

BPO	42901
Nosy	@warsaw, @ethanfurman, @hroncok
PRs	bpo-42901: [Enum] move member creation to __set_name__ #24196

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ethanfurman'
closed_at = <Date 2021-01-25.23:13:30.087>
created_at = <Date 2021-01-12.01:27:28.386>
labels = ['type-feature', 'library', '3.10']
title = '[Enum] move member creation to __set_name__ in order to support __init_subclass__'
updated_at = <Date 2021-02-08.12:41:34.225>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2021-02-08.12:41:34.225>
actor = 'hroncok'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2021-01-25.23:13:30.087>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2021-01-12.01:27:28.386>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 42901
keywords = ['patch']
message_count = 3.0
messages = ['384875', '385006', '386629']
nosy_count = 4.0
nosy_names = ['barry', 'eli.bendersky', 'ethan.furman', 'hroncok']
pr_nums = ['24196']
priority = 'high'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42901'
versions = ['Python 3.10']

[ethanfurman]

In discussions about moving the calls to __set_name__ and __init_subclass__, Nick Coughlan made an observation:

Nick Coghlan:

Both EnumMeta and ABCMeta should probably be relying on __set_name__
for their per-member set up work these days, rather than deferring that
work until after __new__ returns.

By having __set_name__ create the final members, they will be in place for the call to __init_subclass__.

[ethanfurman]

New changeset c314e60 by Ethan Furman in branch 'master':
bpo-42901: [Enum] move member creation to __set_name__ (GH-24196)
c314e60

[hroncok]

I found a possible regression: https://bugs.python.org/issue43162