Require typing_extensions on Python < 3.11

[encukou]

asgi_typing.py has:

if sys.version_info >= (3, 11):
    from typing import NotRequired
else:
    from typing_extensions import NotRequired

However, typing_extensions was not listed as a runtime requirement. If a2wsgi was installed without its test dependencies (which themselves require typing_extensions), it failed to import.

Fixes: #52