tests: switch back to mock instead of backports.unittest_mock

pkg_resources/tests/test_markers.py

-from unittest import mock
+import mock
 
 from pkg_resources import evaluate_marker
 

setuptools/tests/test_build_clib.py

@@ -2,7 +2,7 @@ import pytest
 import os
 import shutil
 
-from unittest import mock
+import mock
 from distutils.errors import DistutilsSetupError
 from setuptools.command.build_clib import build_clib
 from setuptools.dist import Distribution

setuptools/tests/test_easy_install.py

@@ -14,7 +14,7 @@ import itertools
 import distutils.errors
 import io
 import zipfile
-from unittest import mock
+import mock
 
 import time
 from setuptools.extern.six.moves import urllib

setuptools/tests/test_msvc.py

@@ -5,7 +5,7 @@ Tests for msvc support module.
 import os
 import contextlib
 import distutils.errors
-from unittest import mock
+import mock
 
 import pytest
 

tests/requirements.txt

+mock
 pytest-flake8
 pytest>=3.0.2
-# pinned to 1.2 as temporary workaround for #1038
-backports.unittest_mock>=1.2,<1.3