Merge pull request pypa#773 from koobs/patch-1

Fix test command invocation causing ERROR

Author: Ivoz

setup.py

@@ -15,7 +15,7 @@ class PyTest(TestCommand):
 
         def initialize_options(self):
             TestCommand.initialize_options(self)
-            self.pytest_args = None
+            self.pytest_args = []
 
         def finalize_options(self):
             TestCommand.finalize_options(self)