do not hard depend on mock

[pgajdos]

I am not sure whether you need to support all versions on python 3, but in case that yes, could you please consider something like

Index: schedule-1.1.0/test_schedule.py
===================================================================
--- schedule-1.1.0.orig/test_schedule.py        2021-03-07 11:49:24.000000000 +0100
+++ schedule-1.1.0/test_schedule.py     2021-09-08 10:48:44.066906764 +0200
@@ -1,8 +1,11 @@
 """Unit tests for schedule.py"""
 import datetime
 import functools
-import mock
 import unittest
+try:
+    import unittest.mock as mock
+except ImportError:
+    import mock

 # Silence "missing docstring", "method could be a function",
 # "class already defined", and "too many public methods" messages:

[a-detiste]

import unittest.mock as mock should be enough