Enum: improve generics support

GitHub fields:

assignee = 'https://github.com/ethanfurman'
closed_at = None
created_at = <Date 2021-10-29.19:47:03.782>
labels = ['type-feature', '3.11']
title = 'Enum: improve generics support'
updated_at = <Date 2021-10-29.19:47:03.782>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2021-10-29.19:47:03.782>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-10-29.19:47:03.782>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 45676
keywords = []
message_count = None
messages = []
nosy_count = 1.0
nosy_names = ['ethan.furman']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45676'
versions = ['Python 3.11']