Make pytest-runner an optional dependency

setup.py

 # -*- coding: utf-8 -*-
 from __future__ import with_statement
+import sys
 from setuptools import setup
+needs_pytest = {'pytest', 'test', 'ptr'}.intersection(sys.argv)
+pytest_runner = ['pytest-runner'] if needs_pytest else []
+
 def get_version(fname='mccabe.py'):
     with open(fname) as f:
@@ -33,7 +37,7 @@ setup(
     license='Expat license',
     py_modules=['mccabe'],
     zip_safe=False,
-    setup_requires=['pytest-runner'],
+    setup_requires=pytest_runner,
     tests_require=['pytest'],
     entry_points={
         'flake8.extension': [