Simplify batch operations event interpretation

src/python/grpcio/grpc/_cython/_cygrpc/tag.pyx.pxi

@@ -56,18 +56,19 @@ cdef class _BatchOperationTag:
     self._retained_call = call
 
   cdef void prepare(self) except *:
+    cdef Operation operation
     self.c_nops = 0 if self._operations is None else len(self._operations)
     if 0 < self.c_nops:
       self.c_ops = <grpc_op *>gpr_malloc(sizeof(grpc_op) * self.c_nops)
       for index, operation in enumerate(self._operations):
-        (<Operation>operation).c()
-        self.c_ops[index] = (<Operation>operation).c_op
+        operation.c()
+        self.c_ops[index] = operation.c_op
 
   cdef BatchOperationEvent event(self, grpc_event c_event):
+    cdef Operation operation
     if 0 < self.c_nops:
-      for index, operation in enumerate(self._operations):
-        (<Operation>operation).c_op = self.c_ops[index]
-        (<Operation>operation).un_c()
+      for operation in self._operations:
+        operation.un_c()
       gpr_free(self.c_ops)
       return BatchOperationEvent(
           c_event.type, c_event.success, self._user_tag, self._operations)