Properly close aiohttp sessions

ClientSession.close() was always supposed to be awaited on.

Author: agronholm

pusher/aiohttp.py

@@ -40,4 +40,4 @@ def send_request(self, request):
             if response is not None:
                 response.close()
             if session is not None:
-                session.close()
+                yield from session.close()