fix python3.7 compatibility

janus/__init__.py

@@ -16,7 +16,7 @@
 try:
     ensure_future = asyncio.ensure_future
 except AttributeError:
-    ensure_future = asyncio.async
+    ensure_future = getattr(asyncio, 'async')
 
 
 class Queue:

tests/test_async.py

@@ -178,7 +178,7 @@ def test_get_with_putters(self):
 
         @asyncio.coroutine
         def put():
-            t = asyncio.async(q.put(2), loop=self.loop)
+            t = janus.ensure_future(q.put(2), loop=self.loop)
             yield from asyncio.sleep(0.01, loop=self.loop)
             fut.set_result(None)
             return t