[Fix] Mypy (misc) complaints when using mypy's self-typing recommendation for Mixins/Protocols

[nerdai]

Relevant stack overflow:
https://stackoverflow.com/questions/51930339/how-do-i-correctly-add-type-hints-to-mixin-classes/70907644#70907644

Mypy docs/reco for Mixin and Protocol self-typing:
https://mypy.readthedocs.io/en/stable/more_types.html