allow test items to not be associated with a test function

this is needed for plugins like `pytest-pep8` or `pytest-flakes`

_pytest/mark.py

@@ -139,8 +139,9 @@ def matchkeyword(colitem, keywordexpr):
         mapped_names.add(name)
 
     # Add the names attached to the current function through direct assignment
-    for name in colitem.function.__dict__:
-        mapped_names.add(name)
+    if hasattr(colitem, 'function'):
+        for name in colitem.function.__dict__:
+            mapped_names.add(name)
 
     return eval(keywordexpr, {}, KeywordMapping(mapped_names))