drop mock from lower-constraints and requirements

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we don't need it
in lower-constraints and requirements.

These changes will help us to drop `mock` from openstack/requirements

Change-Id: I4d7b11ac4ac5725ad0605b03ddabed22bad8d0d3

test-requirements.txt

@@ -5,4 +5,3 @@ stestr>=2.0.0 # Apache-2.0
 testresources>=2.0.0 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
 ddt>=1.0.1 # MIT
-mock>=2.0.0 # BSD

tests.py

@@ -22,7 +22,7 @@ from ddt import data, ddt, unpack
 from jinja2 import Environment
 from jinja2.exceptions import TemplateRuntimeError
 
-from mock import Mock, patch
+from unittest.mock import Mock, patch
 import os
 import renderspec
 import renderspec.contextfuncs