Migrate from `pydantic` to `attrs`

[astronuttt]

choosing Pydantic was not a wise decision at the start of the project!

it has a lot of overhead, and performance loss and we don't need most of its functionality and things that it brings, since we don't want to do any excessive data validation.

my suggestion is to migrate to attrs before we go any further in the project. I think now is the best time to do this, because if we don't, we may need to change a lot of things in the future.