Switch Python's parsing tech to something more powerful than LL(1)

[njs (Nathaniel J. Smith)]

@guido

>>> f() = 1
  File "<stdin>", line 1
SyntaxError: can't assign to function call
>>> f/ = 1
  File "<stdin>", line 1
    f/ = 1
       ^
SyntaxError: invalid syntax

The two-pass handling needed to catch f() = 1 is definitely ugly, but it does produce a better error message here :-). I don’t have an opinion on whether or not to change the parsing tech, but if we do then I hope high-quality error messages will be given serious weight when picking the replacement.