Add warning when conversion fails with default warn_on_failed_conversion

[sydduckworth]

Description

Part of #2127

• Added is_set function which can be used to check if attributes of an object have been modified along with an _IsSet base class necessary for it to function
• Added asdf.exceptions.AsdfFutureWarning
• Updated file loading logic so that if node conversion fails and warn_on_failed_conversion hasn't been manually set then AsdfFutureWarning is emitted before an exception is raised.

PRs to address downstream test failures:

• Updated tests to prepare for future ASDF changes astropy/asdf-astropy#337
• Update tests to prepare for future ASDF changes spacetelescope/stdatamodels#814

AI Disclosure

No AI tools used

Tasks

• run prek on your machine
• run pytest on your machine
• Does this PR add new features and / or change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • update relevant docstrings and / or docs/ page
  • for any new features, add unit tests

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: bug fix
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.general.rst: infrastructure or miscellaneous change

[braingram]

Thanks for putting this together. I left a few suggestions.

What's your take/plan for the downstream errors?

[braingram]

What are options for consolidating this code with the updates in lazy_nodes to avoid duplicating the error message?

[sydduckworth]

Idk the message is only in two places and will be removed following the next major release, so to me it doesn't seem like a problem. If we really wanted to avoid duplication we could create an AsdfFutureWarning subtype with that text as its message, but that seems like more overhead than the current approach.

[sydduckworth]

@braingram the downstream errors are pretty simple to fix, generally they just require manually setting warn_on_failed_conversion = True in a few tests. I was going to put in an issue about the change for stdatamodels and asdf-astropy.
The downstream changes can be made without needing this PR to actually be merged/released, but I thought it might be good to wait to put the PRs in until this PR is at least approved to demonstrate that the upstream change is actually happening lol.

[braingram]

Thanks! This warning will hopefully inform some downstream users and the docs and details helpfully describe the upcoming changes and how code can be adapted.