use built-in unittest.mock

A built-in unittest.mock module has been introduced in Python 3.3.
Use it instead of depending on external mock package that's only needed
for older Pythons.

Author: mgorny

tests/test_rwx.py

@@ -7,7 +7,7 @@
 from libarchive.entry import format_time
 from libarchive.extract import EXTRACT_OWNER, EXTRACT_PERM, EXTRACT_TIME
 from libarchive.write import memory_writer
-from mock import patch
+from unittest.mock import patch
 import pytest
 
 from . import check_archive, in_dir, treestat

tox.ini

@@ -13,4 +13,3 @@ deps=
     pytest-cov
     pytest-xdist
     six
-    mock