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/generate.py

@@ -3,7 +3,6 @@ import asyncio
 import os
 from pathlib import Path
 import shutil
-import subprocess
 import sys
 from typing import Set