Cherry pick the typeshed fix for 12339

[offbyone]

Description

Typeshed cherry-pick: Use import instead of type alias in email/message.pyi (#7022)

Fixes#12339

(Explain how this PR changes mypy.)

I've confirmed that this resolves the type checking error introduced in 0.940 using hamcrest as a testbed.

[AlexWaygood]

The typeshed fix works fine, but is basically a hack to work around a bug in mypy. @sobolevn worked out how to fix the actual bug in #12134. #12134 is still a work in progress — but maybe it would be good to excise the Any-alias bugfix from that PR, and merge that as a standalone PR, instead of cherry-picking the typeshed hack?

[JelleZijlstra]

If we want to do a bugfix mypy release it's probably easier to merge this simple change.

[AlexWaygood]

If we want to do a bugfix mypy release it's probably easier to merge this simple change.

Yes, fair enough — best to keep it simple.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

Ok, let's use this as a tempory fix.