…ault (RDFLib#1674)

Created method ConjunctiveGraph.serialize(..., format='trig', ...).

Because of typing problems I replaced some annotations in the
overloaded methods of rdflib.Graph.serialize, so that in subclasses
serialize knows, that it returns type(self) and not Graph.

Because Graph.serialize annotations are still incompatible with
ConjunctiveGraph.serialize added type: ignore [overriden]
to let mypy skip that method.