Revision on assignment errors (#97)

* Revision on assignment expression errors

 * added rule for __debug__ (should be a keyword)
 * reviewed error messages
 * added new failing samples

* Adjustment upon Dave's review

 * rewind several changes in assignment errors
 * patched is_definition: command not found for assignment expressions
 * patched Python 2 inconsistent error messages in test_python_errors.py: command not found

parso/python/tree.py

@@ -233,6 +233,8 @@ class Name(_LeafWithoutNewlines):
         while node is not None:
             if node.type == 'suite':
                 return None
+            if node.type == 'namedexpr_test':
+                return node.children[0]
             if node.type in _GET_DEFINITION_TYPES:
                 if self in node.get_defined_names(include_setitem):
                     return node