Fix environment variable usage

Author: nicoddemus

setup.py

@@ -13,7 +13,8 @@
         'pytest>=2.4',
     ],
     extras_require={
-        ':python_version<"3.3"': ['mock'],
+        ':python_version=="2.6" or python_version=="2.7" or '
+        'python_version=="3.2"': ['mock'],
     },
     url='https://github.com/pytest-dev/pytest-mock/',
     license='LGPL',