Make the mock dependency optional

Since Python 3.3 unittest contains the mock module, deprecating the seperate mock module.

Author: jelly

requirements-dev.txt

@@ -2,6 +2,6 @@
 
 coverage
 flake8
-mock
+mock; python_version < '3.3'
 pytest
-tox
+tox

tests/nodeenv_test.py

@@ -7,7 +7,10 @@
 import sys
 import sysconfig
 
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import pytest
 
 import nodeenv

tests/test_install_activate.py

@@ -1,7 +1,10 @@
 import sys
 import os
 
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import pytest
 
 import nodeenv