Fix static type checking for grpclib client (#124)

* Fix static type checking in grpclib client
* Fix python3.6 compatibility issue with dataclasses

src/betterproto/__init__.py

@@ -664,7 +664,8 @@ class Message(ABC):
         """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:
-            field_cls = field_cls.__args__[index]
+            if field_cls.__args__ is not None:
+                field_cls = field_cls.__args__[index]
         return field_cls
 
     def _get_field_default(self, field_name):

src/betterproto/grpc/grpclib_client.py

@@ -13,17 +13,17 @@ from typing import (
     Type,
     Union,
 )
-from .._types import ST, T
+from betterproto._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]]]
-_MessageSource = Union[Iterable["IProtoMessage"], AsyncIterable["IProtoMessage"]]
+_MessageLike = Union[T, ST]
+_MessageSource = Union[Iterable[ST], AsyncIterable[ST]]
 
 
 class ServiceStub(ABC):
@@ -59,7 +59,7 @@ class ServiceStub(ABC):
     async def _unary_unary(
         self,
         route: str,
-        request: "IProtoMessage",
+        request: _MessageLike,
         response_type: Type[T],
         *,
         timeout: Optional[float] = None,
@@ -82,7 +82,7 @@ class ServiceStub(ABC):
     async def _unary_stream(
         self,
         route: str,
-        request: "IProtoMessage",
+        request: _MessageLike,
         response_type: Type[T],
         *,
         timeout: Optional[float] = None,