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.
@@ -5,7 +5,6 @@ openstackdocstheme>=2.2.1 # Apache-2.0
reno>=3.1.0 # Apache-2.0
doc8>=0.6.0 # Apache-2.0
sphinx>=2.0.0,!=2.1.0 # BSD
-mock>=2.0.0 # BSD
os-api-ref>=1.4.0 # Apache-2.0
ddt>=1.0.1 # MIT
fixtures>=3.0.0 # Apache-2.0/BSD
