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

test/test_python_errors.py

@@ -321,4 +321,3 @@ def test_invalid_fstrings(code, message):
 def test_trailing_comma(code):
     errors = _get_error_list(code)
     assert not errors
-