py/compile: Raise SyntaxError if positional args are given after */**.

In CPython 3.4 this raises a SyntaxError.  In CPython 3.5+ having a
positional after * is allowed but uPy has the wrong semantics and passes
the arguments in the incorrect order.  To prevent incorrect use of a
function going unnoticed it is important to raise the SyntaxError in uPy,
until the behaviour is fixed to follow CPython 3.5+.