Avoid 100% CPU usage while socket is closed (#156)

After stop/start kafka service, kafka-python may use 100% CPU caused by
busy-retry while the socket was closed. This fix the issue by unregister
the socket if the fd is negative.

Co-authored-by: Orange Kao <orange@aiven.io>

kafka/client_async.py

@@ -637,6 +637,9 @@ def _poll(self, timeout):
             self._sensors.select_time.record((end_select - start_select) * 1000000000)
 
         for key, events in ready:
+            if key.fileobj.fileno() < 0:
+                self._selector.unregister(key.fileobj)
+
             if key.fileobj is self._wake_r:
                 self._clear_wake_fd()
                 continue