Stricter mypy checks

[ADBond]

Stricter use of mypy:

• removing some excluded modules (i.e. no longer excluded)
• remove a softer checking parameter we had (following imports)
• add in a few stricter checking options (a subset of full --strict mode - see the relevant docs)

Nothing particularly special about the set of options I have added - just following the mypy suggested route, and stopped at a point before the changeset became too large (maybe? sorry if I missed that point!).
We can continue incrementally adding remaining options in follow-ups as and when.

In terms of the code changes to get this up to spec, I don't think there is anything particularly crazy here. Much of it is adding/correcting annotations + renaming variables, but there are a few genuine code changes - don't think anything noteworthy however.

[RobinL]

Great - thanks!