Make email.message.Message generic over the header type

[srittau]

No description provided.

[srittau]

The email/http Message/Header class hierarchy is quite "creative".

[Avasam]

A small adjustment for a comment mentioning the Any trick, otherwise this looks good to me.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spack (https://github.com/spack/spack)
- lib/spack/spack/oci/oci.py:142: error: Argument 1 to "endpoint" of "ImageReference" has incompatible type "str | None"; expected "str"  [arg-type]

[embray]

This seems to have introduced a regression for me at least on Python 3.10, not sure if it works better on say 3.12...

This results in an error:

from email.header import Header
from email.mime.multipart import MIMEMultipart

msg = MIMEMultipart('mixed')
msg['Subject'] = Header('Subject...')

$ mypy --version
mypy 1.11.0 (compiled: yes)
$ mypy .tests/typing/mime.py 
.tests/typing/mime.py:5: error: Incompatible types in assignment (expression has type "Header", target has type "str")  [assignment]

Not clear how this should be typed anymore. If I try to do MIMEMultipart[str, Header] I just get "type[MIMEMultipart]" is not generic and not indexable. I think the default typevars still need to be supplied in the subclasses as well.

I'll set to ignore for now but just as a heads-up.