Fixed mypy issues

Author: syrusakbary

Diff for: graphql/execution/executors/asyncio.py

@@ -37,7 +37,7 @@ def ensure_future(coro_or_future, loop=None):  # type: ignore
     from .asyncio_utils import asyncgen_to_observable, isasyncgen
 except Exception:
 
-    def isasyncgen(obj):
+    def isasyncgen(object):  # type: ignore
         False
 
     def asyncgen_to_observable(asyncgen, loop=None):