unittest: use unittest.mock instead of mock

because the absence of unittest.mock on python2.7, we still fallback to
mock

tests/test_cli/test_upload_progress_callback.py

@@ -1,5 +1,8 @@
 from __future__ import absolute_import
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import six
 import unittest