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: I4ee01710d04d650a3ad5ae069015255d3f674c74

lower-constraints.txt

@@ -53,7 +53,6 @@ Mako==1.0.7
 MarkupSafe==1.0
 mccabe==0.2.1
 microversion_parse==0.2.1
-mock==2.0.0
 monotonic==1.4
 mox3==0.25.0
 msgpack==0.5.6

watcher/tests/decision_engine/datasources/test_manager.py

@@ -16,7 +16,7 @@
 
 from unittest import mock
 
-from mock import MagicMock
+from unittest.mock import MagicMock
 
 from watcher.common import exception
 from watcher.decision_engine.datasources import gnocchi