Use unittest.mock from standard library instead of external mock with Python >=3.3.

setup.py

@@ -170,8 +170,7 @@ setup_params = dict(
     tests_require=[
         'setuptools[ssl]',
         'pytest',
-        'mock',
-    ],
+    ] + (['mock'] if sys.version_info[:2] < (3, 3) else []),
     setup_requires=[
     ] + pytest_runner,
 )

setuptools/tests/fixtures.py

-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import pytest
 
 from . import contexts

setuptools/tests/test_easy_install.py

@@ -15,7 +15,10 @@ import logging
 import itertools
 
 import pytest
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 
 from setuptools import sandbox
 from setuptools import compat

setuptools/tests/test_msvc9compiler.py

@@ -7,7 +7,10 @@ import contextlib
 import distutils.errors
 
 import pytest
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 
 from . import contexts