Fix import order, appeasing lint. (#6240)

Error left from PR #6211, when Kokoro was blithely skipping tests.

Author: tseaver

tests/unit/test_bidi.py

@@ -20,8 +20,8 @@
 import pytest
 from six.moves import queue
 
-from google.api_core import exceptions
 from google.api_core import bidi
+from google.api_core import exceptions
 
 
 class Test_RequestQueueGenerator(object):