switch to python3's buildin mock

Instead of using a python module use the build-in mock and remove the
module from requirements.txt

devel/tests/test_rematch_developers.py

-from mock import patch
+from unittest.mock import patch
 
 
 from django.core.management import call_command

devel/tests/test_reporead.py

 import tarfile
-from mock import patch
+from unittest.mock import patch
 from datetime import datetime
 
 

mirrors/tests/test_mirrorcheck.py

-import mock
 import time
 
+from unittest import mock
 from urllib.error import HTTPError, URLError
 
 

mirrors/tests/test_mirrorresolv.py

-import mock
+from unittest import mock
 
 from django.test import TestCase
 from django.core.management import call_command

requirements.txt

@@ -7,7 +7,6 @@ django-countries==5.0
 jsmin==2.2.2
 pgpdump==1.5
 pytz>=2017.3
-mock==2.0.0
 parse==1.8.2
 django-jinja==2.4.1
 sqlparse==0.2.4