tests: use unittest.mock with py34+

Fixes https://github.com/pytest-dev/pytest/issues/3965.

Has to work around https://github.com/tox-dev/tox/issues/706.

No coverage for pluggymaster builds is OK though anyway.

testing/code/test_code.py

@@ -1,13 +1,19 @@
 # coding: utf-8
 from __future__ import absolute_import, division, print_function
+
 import sys
 
 import _pytest._code
 import pytest
-import mock
-from test_excinfo import TWMock
 from six import text_type
 
+from test_excinfo import TWMock
+
+try:
+    import mock
+except ImportError:
+    import unittest.mock as mock
+
 
 def test_ne():
     code1 = _pytest._code.Code(compile('foo = "bar"', "", "exec"))