pop loop in connection init due to backward compatibility (#808)

* pop loop in connection init due to backward compatibility

* loop pop with default

Author: Dmitry Inuytin

aiopg/connection.py

@@ -75,6 +75,7 @@ def __init__(
         self._waiter = self._loop.create_future()
 
         kwargs['async_'] = kwargs.pop('async', True)
+        kwargs.pop('loop', None)  # backward compatibility
         self._conn = psycopg2.connect(dsn, **kwargs)
 
         self._dsn = self._conn.dsn