Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: Ib6e4e7f89a9990cfb42afa209878812340109ecf
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

heat_cfntools/tests/test_cfn_helper.py

@@ -17,10 +17,10 @@
 import json
 import os
 import tempfile
+from unittest import mock
 
 import boto.cloudformation as cfn
 import fixtures
-import mock
 import testtools
 import testtools.matchers as ttm
 

heat_cfntools/tests/test_cfn_hup.py

@@ -14,9 +14,9 @@
 # under the License.
 
 import tempfile
+from unittest import mock
 
 import fixtures
-import mock
 import testtools
 
 from heat_cfntools.cfntools import cfn_helper

lower-constraints.txt

@@ -1,5 +1,4 @@
 boto==2.32.1
-mock==1.0
 pbr==2.0.0
 psutil==1.1.1
 six==1.9.0

test-requirements.txt

@@ -1,4 +1,3 @@
 hacking>=3.0.1,<3.1.0 # Apache-2.0
-mock>=1.0
 stestr>=2.0.0
 testtools>=0.9.34