Typing issues with Board.backend

[trickeydan]

I think the issue is partially due to backend being of type Type[Backend] rather than
Backend. If the below code is used MyPy is happy but it will almost certainly run time error when we don't provide the necessary values to the subtype's constructor.

board_type: Type['Board'] = backend().board

Converting Backend.board from a @property to a @classmethod is the only thing, other than the cast, that I can think of but that would require large changes to the codebase and Backend.board() doesn't feel particularly nice, though a rename could fix that.

Do you think it's worth changing, or should we just leave as the cast?

Originally posted by @sedders123 in #406

[trickeydan]

Possibly relevant: #342