Fix most mypy warnings

betterproto/_types.py

@@ -1,4 +1,8 @@
-from typing import TypeVar
+from typing import TYPE_CHECKING, TypeVar
+
+if TYPE_CHECKING:
+    from . import Message
+    from grpclib._protocols import IProtoMessage
 
 # Bound type variable to allow methods to return `self` of subclasses
 T = TypeVar("T", bound="Message")