Typing: use wsgiref.types to validate types and fix issues uncovered (#2467)

Typing: use wsgiref.types to validate types and fix issues uncovered (#2467)

* Typing: use wsgiref.types to validate types and fix issues uncovered

- start_response() must return a write(bytes) function, even though this
  is now deprecated. It's fine to be a no-op here.
- sys.exc_info() can return (None, None, None), so make sure to handle that case.

* remove typing_extensions

Co-authored-by: Martijn Pieters <mj@zopatista.com>