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: I6bdf38b8b7e63ff8183f0aaccb566d2baa37df56

os_performance_tools/tests/test_collect.py

@@ -18,8 +18,8 @@ Tests for `os_performance_tools.collect`
 """
 
 import json
-import mock
 import tempfile
+from unittest import mock
 
 from os_performance_tools import collect
 from os_performance_tools.tests import base

os_performance_tools/tests/test_counters2statsd.py

@@ -18,8 +18,8 @@ Tests for `os_performance_tools.counters2statsd`
 """
 
 import json
-import mock
 import time
+from unittest import mock
 
 from os_performance_tools import counters2statsd
 from os_performance_tools.tests import base

os_performance_tools/tests/test_mysql.py

@@ -17,7 +17,7 @@ test_collectors
 Tests for `os_performance_tools.collectors`
 """
 
-import mock
+from unittest import mock
 
 from os_performance_tools.collectors import mysql
 from os_performance_tools.tests import base

os_performance_tools/tests/test_queues.py

@@ -18,7 +18,7 @@ Tests for `os_performance_tools.collectors`
 """
 
 import json
-import mock
+from unittest import mock
 
 from os_performance_tools.collectors import queues
 from os_performance_tools.tests import base

test-requirements.txt

@@ -10,4 +10,3 @@ python-subunit>=0.0.18
 oslotest>=1.10.0 # Apache-2.0
 testrepository>=0.0.18
 testscenarios>=0.4
-mock>=1.2