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: I837241a34505496a3a152d72f28f23687ded1088

doc/requirements.txt

@@ -7,7 +7,6 @@
 # be installed in a specific order.
 #
 # Requirements for docs
-mock>=2.0.0 # BSD
 openstackdocstheme>=2.2.1 # Apache-2.0
 reno>=3.1.0 # Apache-2.0
 sphinx>=2.0.0,!=2.1.0 # BSD

lower-constraints.txt

@@ -39,7 +39,6 @@ jsonschema==2.6.0
 keystoneauth1==3.4.0
 linecache2==1.0.0
 MarkupSafe==1.0
-mock==2.0.0
 monotonic==1.4
 msgpack==0.5.6
 munch==2.2.0

test-requirements.txt

@@ -10,5 +10,4 @@
 hacking>=3.0.1,<3.1.0 # Apache-2.0
 
 coverage!=4.4,>=4.0 # Apache-2.0
-mock>=2.0.0 # BSD
 testtools>=2.3.0 # MIT