Try import mock, but on python 3.3+ can use also stdlib unittest.mock

From Python 3.3 is mock part of python standard library in unittest namespace

AUTHORS

@@ -157,6 +157,7 @@ Oleg Sushchenko
 Oliver Bestwalter
 Omar Kohl
 Omer Hadari
+Ondřej Súkup
 Patrick Hayes
 Paweł Adamczak
 Pedro Algarvio

testing/test_mark.py

@@ -1,7 +1,11 @@
 from __future__ import absolute_import, division, print_function
 import os
 import sys
-import mock
+
+try:
+    import mock
+except ImportError:
+    import unittest.mock as mock
 import pytest
 from _pytest.mark import (
     MarkGenerator as Mark,