Add extra methods to Py::String that as needed on Windows to support full unicode range of code points.
On Windows Python defines Py_UNICODE as unsigned short, which is too small to hold all Unicode values.
PyCXX has added to the Py::String API to support creationg from Py_UCS4 strings and converting Py::String() into Py::ucs4string objects.
Fix validate for Bytes to use the correct check function.