Fix missing import in executors/asyncio

graphql/execution/executors/asyncio.py

@@ -32,18 +32,20 @@ def ensure_future(coro_or_future, loop=None):  # type: ignore
         else:
             raise TypeError("A Future, a coroutine or an awaitable is required")
 
-
 try:
-    from .asyncio_utils import asyncgen_to_observable, isasyncgen
+    from inspect import isasyncgen  # type: ignore
 except Exception:
 
     def isasyncgen(object):  # type: ignore
         False
 
+try:
+    from .asyncio_utils import asyncgen_to_observable
+except Exception:
+
     def asyncgen_to_observable(asyncgen, loop=None):
         pass
 
-
 class AsyncioExecutor(object):
     def __init__(self, loop=None):
         # type: (Optional[_UnixSelectorEventLoop]) -> None

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py34,py35,py36,py37,pre-commit,pypy,mypy,docs
+envlist = py{27,34,35,36,37,py},pre-commit,mypy,docs
 
 [testenv]
 deps =