Drop python3.8 support (#992)

Drop support for soon-to-be eol'd python 3.8.
This will allow some minor cleanups & makes it easier to add support for
timezones.

Related to
https://github.com/python-kasa/python-kasa/issues/980#issuecomment-2170889543

kasa/tests/device_fixtures.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import AsyncGenerator
+from collections.abc import AsyncGenerator
 
 import pytest