Revert "Fix static type checking for grpclib client (danielgtaylor#124)"

This reverts commit beafc81

Author: Gobot1234

src/betterproto/__init__.py

@@ -662,8 +662,7 @@ def _cls_for(cls, field: dataclasses.Field, index: int = 0) -> Type:
         """Get the message class for a field from the type hints."""
         field_cls = cls._type_hint(field.name)
         if hasattr(field_cls, "__args__") and index >= 0:
-            if field_cls.__args__ is not None:
-                field_cls = field_cls.__args__[index]
+            field_cls = field_cls.__args__[index]
         return field_cls
 
     def _get_field_default(self, field_name):

src/betterproto/grpc/grpclib_client.py

@@ -18,14 +18,14 @@
 from .._types import ST, T
 
 if TYPE_CHECKING:
+    from grpclib._typing import IProtoMessage
     from grpclib.client import Channel
     from grpclib.metadata import Deadline
 
 
 _Value = Union[str, bytes]
 _MetadataLike = Union[Mapping[str, _Value], Collection[Tuple[str, _Value]]]
-_MessageLike = Union[T, ST]
-_MessageSource = Union[Iterable[ST], AsyncIterable[ST]]
+_MessageSource = Union[Iterable["IProtoMessage"], AsyncIterable["IProtoMessage"]]
 
 
 class ServiceStub(ABC):
@@ -61,7 +61,7 @@ def __resolve_request_kwargs(
     async def _unary_unary(
         self,
         route: str,
-        request: _MessageLike,
+        request: "IProtoMessage",
         response_type: Type[T],
         *,
         timeout: Optional[float] = None,
@@ -84,7 +84,7 @@ async def _unary_unary(
     async def _unary_stream(
         self,
         route: str,
-        request: _MessageLike,
+        request: "IProtoMessage",
         response_type: Type[T],
         *,
         timeout: Optional[float] = None,