Clean up ReferencedObject typing

[jakebeal]

We are inconsistent about the type declarations regarding ReferencedObject properties.
For example, Constraint uses type Union[Identified,str]:

pySBOL3/sbol3/constraint.py

Lines 19 to 20 in 5787579

	def __init__(self, restriction: str, subject: Union[Identified, str],
	object: Union[Identified, str], *, name: str = None,

while Interface uses types List[str] rather than List[Union[Identified,str]] (or would it be better as Union[List[Identified],List[str])? https://github.com/SynBioDex/pySBOL3/blob/57875793642ca25ad5d5307b85051d8be80e5801/sbol3/interface.py#L13-L14

Since we do in fact allow union types, we should propagate that typing information appropriately.

[tcmitchell]

We could define an ObjectReference to be equivalent to Union[Identified, str] and then use that for the type declaration of variables throughout. Where a referenced object is desired we could then say foo: ObjectReference or bar: List[ObjectReference].

[jakebeal]

This would lose some of the typing information that we currently have, such as in the constructor for Location:

pySBOL3/sbol3/location.py

Line 16 in 6872614

def __init__(self, sequence: Union[Sequence, str],

Is there a way to parameterize this to ObjectReference[Sequence], which would allow the best of both?