rdflib.term._GenericPythonToXSDRules and similar should only take functions that return strings, not binary.

[aucampia]

Currently it takes functions returning string or bytes:

rdflib/rdflib/term.py

Lines 1955 to 1957 in 394215f

	_GenericPythonToXSDRules: List[
	Tuple[Type[Any], Tuple[Optional[Callable[[Any], Union[str, bytes]]], Optional[str]]]
	] = [

This is an example of a function returning bytes:

rdflib/rdflib/term.py

Lines 1657 to 1659 in 394215f

	def _writeXML( # noqa: N802
	xmlnode: Union[xml.dom.minidom.Document, xml.dom.minidom.DocumentFragment]
	) -> bytes:

I think this adds uncessary complexity and it would be best to have the functions always return either string or bytes.