Add missing dependency on py

Author: swt2c

setup.py

@@ -19,7 +19,7 @@
         ],
     },
     zip_safe=False,
-    install_requires=['pytest>=3.1.0'],
+    install_requires=['py', 'pytest>=3.1.0'],
     setup_requires=['setuptools_scm'],
     python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*',
     classifiers=[