tests: target Python 3.8 as the minimum Python version

This version is already two releases out of support, but is "only" 7
years old so is probably still being used in the real world. Document
this version along with some other testing dependencies.  Remove code
support for earlier versions. Disable ruff checks that need a newer
version.

tests/http/testenv/ports.py

@@ -25,8 +25,7 @@
 import logging
 import os
 import socket
-from collections.abc import Callable
-from typing import Dict
+from typing import Callable, Dict
 
 from filelock import FileLock