Add MyPy checks for typing consistency.

[HGWright]

✨ Feature Request

Motivation

As part of #5924.

Within the MyPy section of pyproject.toml, introduce MyPy checks for typing consistency, i.e. disallow_untyped_calls, disallow-incomplete-defs and disallow-untyped-decorators. Consider if any other MyPy checks are worth adding.

• Also consider what to do with the existing strict flag, as it may or may not clash with the specific flags below. At the very least it may confuse future developers?

Additional context

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[bouweandela]

I like the approach of setting strict = true and disabling checks that cannot yet pass with a TODO message. This ensures that new checks are automatically enabled as newer versions with more checks are released. There is some good advice on how to adopt mypy with an existing code base available here: https://mypy.readthedocs.io/en/stable/existing_code.html#existing-code