`email.policy.default` doesn't work with `email.parser.BytesParser` or `email.parser.Parser`

[dseomn]

I just started getting a mypy error that I think is caused by the same issue, but it doesn't look like this PR touches email.parser.BytesParser at all. Would it make sense to change that in this PR too? Or should I file a separate bug?

Error:

rock_paper_sand/report_test.py:465: error: Argument "policy" to "BytesParser" has incompatible type "EmailPolicy[EmailMessage]"; expected "Policy[Message[str, str]]"  [arg-type]

Code:

        message = typing.cast(
            email.message.EmailMessage,
            email.parser.BytesParser(policy=email.policy.default).parsebytes(
                mock_subprocess_run.mock_calls[0].kwargs["input"]
            ),
        )

Originally posted by @dseomn in #13274 (comment)

mypy 1.15.0 also complains about parser.Parser(policy=policy.default) so i continue to use 1.13.0 and changed my code to use message_from_string in anticipation of this PR's deployment. happy to see more commits, but merging this PR as-is would be a step in the right direction.

Le mieux est l'ennemi du bien.

Originally posted by @cossor in #13274 (comment)