Post-review of transition to `attrs`

[inducer]

After merging #365, I noticed a few things I wish I'd flagged, see #365 (review). They're not a huge deal, but I think we should address them to avoid setting traps for our future selves.

[kaushikcfd]

From #371 (comment), automaticall populate _fields.

[matthiasdiener]

Out of curiosity, should we now consider moving back to dataclasses (which would imply Python 3.10+)?

[inducer]

In general, I'm in favor of that. Good reasons to go back:

• uniformity? Everywhere else in the stack uses dataclasses.
• https://docs.python.org/3/library/typing.html#typing.dataclass_transform would allow us to define a single class decorator for array-likes, vs. the inconsistent mess that we have currently.

Obstacles:

• We're using cache_hash=True in attrs, for which dataclasses has no equivalent. We would need to generate our own code for __hash__. Could be worse, I think.