Allow derived classes for str and tuple in WSGI validator

Feature or enhancement

The WSGI validator could allow anything that inherits from tuple[str, str] for the headers, instead of literally just tuple[str, str] and nothing else.

Pitch

This is so I can use classes that extend str and tuple[str, str] as headers.
Symbolic types that add essentially nothing but a pretty constructor and type checks avoid silly mistakes by encoding intent.

Previous discussion

On IRC I was told about contravariance. https://peps.python.org/pep-0484/#covariance-and-contravariance
I think it might be fine, because no matter which class extending its type I will feed it, it's never coming back, so there is no risk of an item of a less precise type bubbling up to the caller somehow.

Linked PRs

• gh-93640

Commencez par localiser le point d’entrée du validateur WSGI et examinez le PR lié gh-93640 avant de commencer, puisque l’issue indique qu’un travail connexe est effectué ailleurs. Vérifiez que le validateur accepte les sous-classes de str et tuple[str, str] tout en conservant son comportement actuel de rejet ; c’est terminé lorsque les classes de headers dérivées prévues sont acceptées et que le comportement existant du validateur reste couvert.