Fixes an issue where the newer versions of Pytest can't detect deprecation warnings

This was fixed in pytest-dev/pytest#1081 but is not released yet.

Author: mark-adams

setup.py

@@ -30,7 +30,7 @@ def get_version(package):
     sys.exit()
 
 tests_require = [
-    'pytest',
+    'pytest==2.7.3',
     'pytest-cov',
     'pytest-runner',
 ]