In CPython, PyString_GET_SIZE and PyUnicode_GET_SIZE happen to have
the same implementation, so it is ok for callers to be wrong about
which one they call.

For us, previous to this commit, PyString_GET_SIZE would call PyString_Size,
which has different behavior than PyString_GET_SIZE for non-strings.
For unicode objects it means that we would return the size of the encoded
representation of the string rather than the number of unicode characters.
It also means that PyString_GET_SIZE can fail for us and return -1,
which it never can for CPython, so callers get away with not
checking the return value.