Address disabled MyPy checks

[pp-mo]

QA checks which we expect later to review + possibly remove.

Controls are in pyproject.toml here

Summary of controls

The ultimate goal would be to have no 'ignores', but there are some files excluded.

We have set "strict=true", but some of the group of checks which that enables (see here) are temporarily turned off.
It is noted that setting 'strict=True' may be uncertaion does may change with future releases,
but we will probably decide that some exceptions will remain "permanent".

Extra non-default checks permanently enabled :

• warn_unused_configs
• warn_unreachable
• enable_error_code = ["ignore-without-code", "truthy-bool"]

Remaining temporarily disabled checks :

• disallow_any_generics
• disallow_subclassing_any
• disallow_untyped_calls
• disallow_untyped_defs
• disallow_incomplete_defs
• check_untyped_defs
• no_implicit_reexport