Correct documentation and type hints where functions may return None

[elliot-100]

This PR makes several changes to type hints and docstrings. No run-time checking is done and there should be zero runtime impact.

• Modernises existing Optional[Type] pattern hints to Python >= 3.10 Type | None pattern
• Refactors type hints, by extracting a very simple JSONDict type alias to be used for dicts to be passed to/from JSON. It's equivalent to Dict[str, Any].

Together these mean that e.g. Optional[list[Dict[str, Any]]] can be written more simply, as list[JSONDict] | None.

• Adds list[JSONDict] | None hint to relevant Spond, SpondClub attributes. Also adds relevant JSONDict hints to test fixtures and parameters.

NB: This does not add JSONDict hints to returns from Spond.send_message() or Spond.update_event() as these need a bit more thought - both seem to have oddities in what they currently return.