Message 244208 - Python tracker

Berker's patch looks good.
It has several virtues:

* the error message is reasonable and clear
* it makes the language more consistent
* it doesn't break any existing code.
* it makes the AST a little simpler and faster
  by removing a special case

The patch needs to updated:
* remove the whatsnew entry
* fix test_codeop which expects "del ()" to raise SyntaxError
* fix test_syntax which fails on "() = 1" and "del ()"
* fix test_with which fails on "with mock as ()"