Change AsyncioExecutor loop argument type (#273)

Author: blazewicz

graphql/execution/executors/asyncio.py

@@ -6,7 +6,7 @@
 
 # Necessary for static type checking
 if False:  # flake8: noqa
-    from asyncio.unix_events import _UnixSelectorEventLoop
+    from asyncio import AbstractEventLoop
     from typing import Optional, Any, Callable, List
 
 try:
@@ -51,7 +51,7 @@ def asyncgen_to_observable(asyncgen, loop=None):
 
 class AsyncioExecutor(object):
     def __init__(self, loop=None):
-        # type: (Optional[_UnixSelectorEventLoop]) -> None
+        # type: (Optional[AbstractEventLoop]) -> None
         if loop is None:
             loop = get_event_loop()
         self.loop = loop