Use mock from the standard library

Since Python 3.3, mock is part of unittest in the standard library.

Provide compatibility for older versions, since httplib2 seems to still
support Python2.

requirements-test.txt

@@ -1,6 +1,6 @@
 flake8==3.4.1
 future==0.16.0
-mock==2.0.0
+mock==2.0.0;python_version<"3.3"
 pytest-cov==2.5.1
 pytest-forked==0.2
 pytest-randomly==1.2.1

tests/test_cacerts_from_env.py

@@ -1,6 +1,9 @@
 import os
 import sys
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import pytest
 import tempfile
 import httplib2

tests/test_http.py

@@ -5,7 +5,10 @@
 import email.utils
 import errno
 import httplib2
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import os
 import pytest
 from six.moves import http_client, urllib

tests/test_other.py

@@ -1,5 +1,8 @@
 import httplib2
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import os
 import pickle
 import pytest

tests/test_proxy.py

@@ -9,7 +9,10 @@
 from __future__ import print_function
 
 import httplib2
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import os
 import pytest
 import socket