Use print() function in both Python 2 and Python 3 (#722)

* Use print() function in both Python 2 and Python 3

Legacy __print__ statements are syntax errors in Python 3 but __print()__ function works as expected in both Python 2 and Python 3.

* Fix undefined names
diff --git a/googleapiclient/http.py b/googleapiclient/http.py
index e795274..a57f83d 100644
--- a/googleapiclient/http.py
+++ b/googleapiclient/http.py
@@ -156,7 +156,7 @@
       LOGGER.warning(
           'Sleeping %.2f seconds before retry %d of %d for %s: %s %s, after %s',
           sleep_time, retry_num, num_retries, req_type, method, uri,
-          resp.status if resp else exception)
+          resp.status if resp else Exception)
       sleep(sleep_time)
 
     try: