ttlser fails if a literal is a subject

[tgbugs]

import rdflib
from pyontutils.core import makeGraph
g = makeGraph('buggraph', graph=rdflib.Graph())
g.g.add((rdflib.Literal('UT OH'), rdflib.RDFS.comment, rdflib.Literal('This will end badly.')))
g.write()

Produces a key error since subject literals don't seem to make it into the global sort keys.

The default turtle serializer produces the following.

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xml: <http://www.w3.org/XML/1998/namespace> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

"UT OH" rdfs:comment "This will end badly." .

[tgbugs]

Related to RDFLib/rdflib#895.

[tgbugs]

The issue is in CustomTurtleSerializer._globalSortKey where, not surprisingly, a literal value is not ranked, because there should not be any literal values there. I'm ruling that this is not a bug because it is consistent with the RDF spec, even though it is not consistent with rdflib behavior. The current behavior is hard to debug, because a bad value can creep in e.g. via a parse because rdflib is too permissive, and only later will the error appear, however, since these are literals in the subject position, they shouldn't be particularly difficult to track down.