Oops. Python 2.6 doesn't have set literals

setup.py

@@ -4,7 +4,7 @@ import sys
 from setuptools import setup
-needs_pytest = {'pytest', 'test', 'ptr'}.intersection(sys.argv)
+needs_pytest = set('pytest', 'test', 'ptr').intersection(sys.argv)
 pytest_runner = ['pytest-runner'] if needs_pytest else []