catch asyncio.CancelledError as well as concurrent.futures.CancelledError

as of [bpo-32528](https://bugs.python.org/issue32528), asyncio.CancelledError is
not a subclass of concurrent.futures.CancelledError. This means that if an
asyncio future raises an exception, it won't be caught. Therefore, the
exception will bubble past the try-except within the loop in application_checker,
resulting in done applications not being cleaned up, and the application_checker
task not being queued again.

daphne/server.py

@@ -277,7 +277,7 @@ class Server(object):
             if application_instance and application_instance.done():
                 try:
                     exception = application_instance.exception()
-                except CancelledError:
+                except (CancelledError, asyncio.CancelledError):
                     # Future cancellation. We can ignore this.
                     pass
                 else: