drop Python 3.8 support (#10265)

* drop Python 3.8

Note that this PR only makes typing related changes.
Other changes can be done once this gets merged, they
are relatively few in numbers.

* bump build python version

* add typing_extensions to lint group

tests/utils/asserts.py

@@ -1,11 +1,11 @@
-from typing import TYPE_CHECKING, Any, Dict
+from typing import TYPE_CHECKING, Any
 from unittest.mock import ANY
 
 if TYPE_CHECKING:
     from unittest.mock import Mock
 
 
-def issubset(subset: Dict, superset: Dict) -> bool:
+def issubset(subset: dict, superset: dict) -> bool:
     assert superset == {**superset, **subset}
     return True