Rm attrs

[Jelleas]

Per @cmlsharp's comment in cs50/check50#368, Python 3.7 brought with it dataclasses that can directly replace attrs. This pr replaces the only use of attrs in lib50 with dataclasses and removes attrs as a dependency. lib50 already requires python>=3.8, so no change needed there.

slots can be set through dataclasses too, but requires python>=3.10 (see https://docs.python.org/3/library/dataclasses.html#module-contents). Functionally this is not needed for lib50.

[cmlsharp]

👍 on slots not being necessary. I think pre-3.10 you could still declare __slots__ on a dataclass, but IIRC some stuff like default values wouldn't work out of the box. The memory usage/performance of lib50 should in no way bottlenecked by the User struct so it's not a big deal not to have them if you want the min supported version to be < 3.10