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: Ic1a687f95e5fd16ffa7f63102d6f8f2de383780e

test-requirements.txt

@@ -2,6 +2,5 @@
 flake8
 os-testr>=0.4.1
 charms.reactive
-mock>=1.2
 coverage>=3.6
 git+https://github.com/openstack/charms.openstack.git#egg=charms.openstack

unit_tests/test_requires.py

@@ -12,7 +12,7 @@
 
 
 import unittest
-import mock
+from unitest import mock
 
 import requires