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: If307ca1f689b1ade1428b904bbf122bd6aa5da97

ironic_lib/tests/test_basic_auth.py

@@ -16,8 +16,7 @@
 import base64
 import os
 import tempfile
-
-import mock
+from unittest import mock
 
 from ironic_lib import auth_basic
 from ironic_lib import exception

lower-constraints.txt

@@ -20,7 +20,6 @@ iso8601==0.1.11
 keystoneauth1==3.4.0
 linecache2==1.0.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==0.6
 mox3==0.20.0
 msgpack-python==0.4.0