extract linewriter into a context manager

[graingert]

#1695 is needed for the multiline cmgr expression here:

        cmgr: (
            contextlib.AbstractContextManager[_DryRunWriter]
            | contextlib.AbstractContextManager[_LineWriter]
        ) = (
            contextlib.nullcontext(_DryRunWriter())
            if self.dry_run
            else _LineWriter.create(buffer=self.dst_file, newline=self.linesep)
        )

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[graingert]

the actual change for this PR is here 55bcc80 (#1696)

[graingert]

closing in favour of #1698