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

Start by locating the WSGI validator entry point and inspect linked PR gh-93640 before beginning, since the issue indicates related work elsewhere. Confirm whether the validator accepts subclasses of str and tuple[str, str] while preserving its current rejection behavior; done means the intended derived header classes are accepted and the existing validator behavior remains covered.