Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I2b8d8d6cf4ac29dc58cd8fe418d613445d65c3be
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

futurist/tests/test_periodics.py

@@ -16,10 +16,10 @@ import contextlib
 import functools
 import threading
 import time
+from unittest import mock
 
 import eventlet
 from eventlet.green import threading as green_threading
-import mock
 import testscenarios
 
 import futurist