Make `Settings` attributes warn on *assign*, not on *access* (or reset)

[flying-sheep]

Description of feature

Settings get read internally and assigning them is the user facing API.

Therefore pydantic’s behavior of warning on access (which is also triggered by reset btw.) but not on assignment doesn’t fit and we should change that.

This tripped up @ilan-gold and me in scverse/anndata#2643

[ilia-kats]

I think that would fit well as part of pydantic/pydantic#8922. Should we try to implement this upstream, e.g. as a model_config option deprecation_warning_on: Literal["get", "set", "get_and_set"]?

[flying-sheep]

Good point! this comment asks exactly for direction about that and didn’t get an answer: pydantic/pydantic#8922 (comment)

So if we can get hold of Pydantic folks and some clear go-ahead that this is wanted (and that a good design exists) then yeah, otherwise I feel like it’ll take a while.

But it’s not super important anyway. I think if we suppress the warning on reset right now, we’re in a good state too, since people probably set settings before using them and then see the warning.

[ilia-kats]

The Pydantic changes required to make this work are actually fairly minimal: pydantic/pydantic#13797.