Use unittest.mock instead of third party mock

We can now use the standard library mock library instead of the third
party one previously needed for py27 compatibility.

Change-Id: I6b80b62d8e3c8bd9432f567e0725c191cef6f715
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

reviewstats/tests/test_projectinfo.py

@@ -10,7 +10,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import mock
+from unittest import mock
 
 from reviewstats.tests import base
 from reviewstats import utils

test-requirements.txt

@@ -4,4 +4,3 @@ coverage>=3.6
 fixtures>=0.3.14
 python-subunit
 stestr>=2.0.0
-mock