do not hard depend on mock

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:

```