tox: drop mock dep since we only support py versions with mock included

test/test_vimball.py

@@ -1,9 +1,8 @@
 import os
 import stat
 from tempfile import NamedTemporaryFile, TemporaryDirectory
-from unittest import mock
+from unittest.mock import patch
 
-from mock import patch
 from pytest import raises
 
 from vimball import Vimball, mkdir_p, is_vimball, ArchiveError

tox.ini

@@ -7,7 +7,6 @@ setenv =
 	PYTHONPATH = {envsitepackagesdir}
 deps =
 	pytest
-	mock
 	coverage
 commands =
 	coverage run --source vimball -m py.test {toxinidir}/test