[ast27] Fix PyBytes_AS_STRING being called on unicode objects

[ddfisher]

This lets typed_ast run on debug-mode Python. I believe this also a minor correctness fix.

[gvanrossum]

This doesn't look correct. PyUnicode_AsUTF8String() returns a PyObject *, not a C string (https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_AsUTF8String), so you're casting an object to (char *) which is then treated as a C string by forbidden_check(). So then the latter will always return 1.

The original code used PyBytes_AS_STRING() which does return a C string.