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/_types.py

@@ -2,7 +2,7 @@ from typing import TYPE_CHECKING, TypeVar
 
 if TYPE_CHECKING:
     from . import Message
-    from grpclib._protocols import IProtoMessage
+    from grpclib._typing import IProtoMessage
 
 # Bound type variable to allow methods to return `self` of subclasses
 T = TypeVar("T", bound="Message")