Minor non-functional changes

- fix few typos
- remove unused imports
- fix minor code-quality issues
- replace `grpclib._protocols` with `grpclib._typing`
- fix boolean and None assertions in test cases

betterproto/tests/grpc/thing_service.py

@@ -3,10 +3,10 @@ from betterproto.tests.output_betterproto.service.service import (
     DoThingRequest,
     GetThingRequest,
     GetThingResponse,
-    TestStub as ThingServiceClient,
 )
 import grpclib
-from typing import Any, Dict
+import grpclib.server
+from typing import Dict
 
 
 class ThingService: