dataclasses: it should be an error to specify KW_ONLY twice

[ericvsmith]

BPO	44015
Nosy	@ericvsmith
PRs	bpo-44015: dataclasses should allow KW_ONLY to be specified only once per class #25841

^{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/ericvsmith'
closed_at = <Date 2021-05-03.07:26:01.594>
created_at = <Date 2021-05-03.06:09:50.431>
labels = ['type-bug', 'library', '3.10']
title = 'dataclasses: it should be an error to specify KW_ONLY twice'
updated_at = <Date 2021-05-03.07:26:01.594>
user = 'https://github.com/ericvsmith'

bugs.python.org fields:

activity = <Date 2021-05-03.07:26:01.594>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2021-05-03.07:26:01.594>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2021-05-03.06:09:50.431>
creator = 'eric.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 44015
keywords = ['patch']
message_count = 2.0
messages = ['392768', '392772']
nosy_count = 1.0
nosy_names = ['eric.smith']
pr_nums = ['25841']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44015'
versions = ['Python 3.10']

[ericvsmith]

Specifying KW_ONLY twice should raise an exception, but does not:

>>> @dataclasses.dataclass
... class foo:
...   _: dataclasses.KW_ONLY
...   x: dataclasses.KW_ONLY
...

[ericvsmith]

New changeset 99ad742 by Eric V. Smith in branch 'master':
bpo-44015: dataclasses should allow KW_ONLY to be specified only once per class (GH-25841)
99ad742