Remove mock as requirement as now part of Py 3.3+

Remove mock as a test requirement as it's now part of Python from 3.3
onwards and we're dropping the last bits of Python 2 support. Unit test
updated to use new import (but maintain rest of code).

requirements-test.txt

@@ -1,6 +1,5 @@
 # Additional requirements for the unit and coverage tests
 
 coveralls<=1.2.0
-mock<4.0.0  # Pinned because version 4 dropped support for Python 2.7
 codecov
 pre-commit

test/test_agents.py

@@ -4,8 +4,7 @@
 import tempfile
 from textwrap import dedent
 import unittest
-
-import mock
+import unittest.mock as mock
 
 import ssm.agents
 from ssm.ssm2 import Ssm2Exception