Make InvalidVarException._get_origin a staticmethod (#482)

Author: blueyed

pytest_django/plugin.py

@@ -488,7 +488,8 @@ def __contains__(self, key):
             """There is a test for '%s' in TEMPLATE_STRING_IF_INVALID."""
             return key == '%s'
 
-        def _get_origin(self):
+        @staticmethod
+        def _get_origin():
             stack = inspect.stack()
 
             # Try to use topmost `self.origin` first (Django 1.9+, and with