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

先定位 WSGI validator 的入口点，并在开始之前检查关联的 PR gh-93640，因为 issue 表明其他地方存在相关工作。确认 validator 是否接受 str 的子类和 tuple[str, str]，同时保留其当前的拒绝行为；当预期的派生 header 类能够被接受，并且现有的 validator 行为仍得到覆盖时，即视为完成。