Use unittest.mock instead of mock

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we can use the
standard lib unittest.mock module instead.

Change-Id: I887adac6caebb327871d3709ed59f25e401b5600

test-requirements.txt

@@ -2,6 +2,5 @@
 flake8
 os-testr>=0.4.1
 charms.reactive
-mock>=1.2
 coverage>=3.6
 netifaces

unit_tests/test_provides.py

@@ -12,7 +12,7 @@
 
 
 import unittest
-import mock
+from unitest import mock
 
 
 with mock.patch('charmhelpers.core.hookenv.metadata') as _meta:

unit_tests/test_requires.py

@@ -12,7 +12,7 @@
 
 
 import unittest
-import mock
+from unitest import mock
 
 
 with mock.patch('charmhelpers.core.hookenv.metadata') as _meta: