Evaulate attrs change to attribute setting

[madig]

See python-attrs/attrs#886.

The docs say that @defined dataclasses will change behavior:

Converters and validators are run when you set an attribute (on_setattr=[attr.setters.convert, attr.setters.validate]).

After a version with this change is released, we should go over the code and see if we need to change anything or can simplify something.

[anthrotype]

we could but mypy can't #188