use the new unittest.mock as integrated in the standard library

.github/workflows/python-package.yml

@@ -26,7 +26,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install flake8 pytest mock
+        python -m pip install flake8 pytest
         python -m pip install -e .
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
     - name: Lint with flake8

test-requirements.txt

@@ -1,2 +1 @@
 pytest<5
-mock<3

tests/test_cache.py

@@ -1,6 +1,6 @@
 import json
 import os
-import mock
+from unittest import mock
 import unittest
 from pathlib import Path