fix(relations): remove some typevars in favor of Any

[Linkid]

I have made things!

RelatedField subclasses can avoid dealing with data type and model type TypeVars override, as they are not specific.

This should fix the "Need type annotation for" that mypy raises for all related field subclasses.

Related issues

Ref #168

[Linkid]

Hello. I've forgotten some methods. This should be fixed in the latest commit.