Add minimal typing support and fix py3.10

[JacobHayes]

Changes from inspect.Parameter.annotation to typing.get_type_hints in order to accommodate string annotations from:

• explicit string hints
• PEP 563: default in py >=3.10, from __future__ import annotations in py <3.9

Also adds some minimal type hints to Dispatcher to ease mypy typing in downstream use - happy to separate this part.

Resolves #104