tests: use unittest.mock with py34+

Fixes #3965.

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"))

tox.ini

@@ -30,15 +30,15 @@ setenv =
 deps =
     hypothesis>=3.56
     nose
-    mock
+    py27: mock
     requests
     {env:_PYTEST_TOX_EXTRA_DEP:}
 
 [testenv:py27-subprocess]
 changedir = .
 deps =
     pytest-xdist>=1.13
-    mock
+    py27: mock
     nose
 passenv = USER USERNAME TRAVIS
 commands =
@@ -54,7 +54,7 @@ commands = pre-commit run --all-files --show-diff-on-failure
 [testenv:py27-xdist]
 deps =
     pytest-xdist>=1.13
-    mock
+    py27: mock
     nose
     hypothesis>=3.56
     {env:_PYTEST_TOX_EXTRA_DEP:}
@@ -87,7 +87,7 @@ commands = {[testenv:py27-pexpect]commands}
 deps =
     pytest-xdist>=1.13
     hypothesis>=3.56
-    mock
+    py27: mock
     {env:_PYTEST_TOX_EXTRA_DEP:}
 distribute = true
 changedir=testing