grpc _CallIterator is an iterator (#13925)

Author: macro1

stubs/grpcio/@tests/test_cases/check_grpc.py

@@ -44,3 +44,4 @@
 call_iter = cast(grpc._CallIterator[str], None)
 for call in call_iter:
     assert_type(call, str)
+assert_type(next(call_iter), str)

stubs/grpcio/grpc/__init__.pyi

@@ -426,6 +426,7 @@ class UnaryUnaryClientInterceptor(abc.ABC, Generic[_TRequest, _TResponse]):
 @type_check_only
 class _CallIterator(Call, Generic[_TResponse], metaclass=abc.ABCMeta):
     def __iter__(self) -> Iterator[_TResponse]: ...
+    def __next__(self) -> _TResponse: ...
 
 class UnaryStreamClientInterceptor(abc.ABC, Generic[_TRequest, _TResponse]):
     @abc.abstractmethod