Use PEP-0426 environment markers to build universal wheels

PEP-0426 defines [environment markers][] to allow wheels to
conditionally install dependencies. This won't work with really old
versions of setuptools/pip, but it works with pip 1.5.6 and newer.

[environment markers]: https://www.python.org/dev/peps/pep-0426/#environment-markers

Author: Andy Freeland

setup.cfg

@@ -0,0 +1,2 @@
+[bdist_wheel]
+universal=1

setup.py

@@ -1,12 +1,6 @@
-import sys
-
 from setuptools import setup
 
 
-dependencies = ['pytest>=2.4']
-if sys.version_info < (3, 3):
-    dependencies.append('mock')
-
 setup(
     name='pytest-mock',
     version='0.7.0',
@@ -15,7 +9,12 @@
     },
     py_modules=['pytest_mock'],
     platforms='any',
-    install_requires=dependencies,
+    install_requires=[
+        'pytest>=2.4',
+    ],
+    extras_require={
+        ':python_version<"3.3"': ['mock'],
+    },
     url='https://github.com/pytest-dev/pytest-mock/',
     license='LGPL',
     author='Bruno Oliveira',