Improve error message in enum for member name surrounded by underscore.

[RubnJessGarcaHernndez]

BPO	38967
Nosy	@ethanfurman, @miss-islington, @tirkarthi
PRs	bpo-38967: Improve the error msg for reserved _sunder_ names in enum #18370
Files	enum.diff.txt: Patch for enum.py

^{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 2020-09-13.20:36:47.981>
created_at = <Date 2019-12-04.09:06:38.910>
labels = ['type-feature', 'library', '3.10']
title = 'Improve error message in enum for member name surrounded by underscore.'
updated_at = <Date 2020-09-13.20:36:47.980>
user = 'https://bugs.python.org/RubnJessGarcaHernndez'

bugs.python.org fields:

activity = <Date 2020-09-13.20:36:47.980>
actor = 'ethan.furman'
assignee = 'ethan.furman'
closed = True
closed_date = <Date 2020-09-13.20:36:47.981>
closer = 'ethan.furman'
components = ['Library (Lib)']
creation = <Date 2019-12-04.09:06:38.910>
creator = 'Rub\xc3\xa9n Jes\xc3\xbas Garc\xc3\xada Hern\xc3\xa1ndez'
dependencies = []
files = ['48754']
hgrepos = []
issue_num = 38967
keywords = ['patch']
message_count = 5.0
messages = ['357782', '357783', '359059', '376845', '376846']
nosy_count = 5.0
nosy_names = ['docs@python', 'ethan.furman', 'miss-islington', 'xtreak', 'Rub\xc3\xa9n Jes\xc3\xbas Garc\xc3\xada Hern\xc3\xa1ndez']
pr_nums = ['18370']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38967'
versions = ['Python 3.10']

[RubnJessGarcaHernndez]

I changed the '_names_ are reserved for future Enum use' line to be more user-friendly thus:
'Names surrounded by underscore (such as "%s") are reserved for future Enum use' % key

The current message can be interpreted as the literal string _names_; and showing the offending key can help users debug.

[tirkarthi]

Error message changes in code are normally not backported to older versions so I have changed the subject accordingly and tagged 3.9. Thanks.

[ethanfurman]

Rubén, good idea.

I wonder, though, if we should say:

_sunder_ names, such as "%s", are reserved for future Enum use

since _sunder_ is used in the Enum docs and is unlikely to be confused with one's own variable name.

[miss-islington]

New changeset 2ec6752 by Zackery Spytz in branch 'master':
bpo-38967: Improve the error msg for reserved _sunder_ names in enum (GH-18370)
2ec6752

[ethanfurman]

Thank you, Rubén, for the patch.

Thank you, Karthikeyan, for not making me backport it. :-)