Following openstack horizon change
I2de669d8e89b8daeb7ee5405ffab35af6307c40b
Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.
Unblocks currently broken pyX gates
Change-Id: Id140c0a13c9d2a2bfcd1d851a04407f232021eef
Closes-Bug: #1870436