[attrs] Field level kw_only=False overrides kw_only=True at class level (#20949)
fixes #20947
When setting kw_only=True at class level with attrs, all fields are set
to be keyword only. Since attrs=v25.4.0, individual attributes with
kw_only=False are treated as positional (to be consistent with
dataclasses, see https://github.com/python-attrs/attrs/pull/1457).
With this PR, Mypy considers attributes with kw_only=False to be
positional.
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