A bit of a nitpick, but the following SyntaxError message is a bit confusing:

>>> f(True=1)
  File "<stdin>", line 1
    f(True=1)
      ^^^^^
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?

The problem with that line is not that it contains an assignment, it's almost a valid keyword argument after all. The problem is that the name of the keyword is True, which is no longer a name you can assign to. It would be better to produce the same error as with __debug__:

>>> f(__debug__=1)
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__

The latter error message is however produced by the compiler, not the parser I think?