No type hints for ProvDocument.agent etc.

[Joe-Heffer-Shef]

It would be easier to use the code if type hints were used here
https://github.com/trungdong/prov/blob/master/src/prov/model.py#L1829
like this:

def agent(self, identifier, other_attributes=None) -> ProvAgent:

[trungdong]

Thanks for the suggestion. This is next on my plan for version 2.1. I'm in fact half way through it at the moment. I hope to get it done and released soon.