issue2551296 - from mock import Mock?

Fix mock import for newer python3 to use unittest.mock rather than
mock.

Author: rouilj

CHANGES.txt

@@ -62,6 +62,9 @@ Fixed:
 - Handle out of memory error when importing large trackers in
   PostgreSQL. (Found by Norbert Schlemmer, extensive testing by
   Norbert, fix John Rouillard)
+- use unittest.mock rather than mock for
+  test/test_hyperdbvals.py. (found by Ralf Schlatterbeck. Fix John
+  Rouillard)
 
 Features:
 

test/test_hyperdbvals.py

@@ -10,7 +10,11 @@
 
 import unittest, os, shutil, errno, sys, difflib, re
 from hashlib import sha1
-from mock import Mock
+try:
+   from unittest.mock import Mock
+except ImportError:
+   # python 2.7
+   from mock import Mock
 
 from roundup import init, instance, password, hyperdb, date