Final 4.0.0 updates

[braingram]

Adds 'convert_unknown_ndarray_subclasses' change to what's new:
Closes #1865

Adds a check if the currently used git commit is tagged to the readthedocs configuration to close #1844
For an untagged commit towncrier build --keep will be called.
For a tagged commit towncrier build --keep will be skipped.

Updates changelog for 4.0.0.

Romancal regtests: https://github.com/spacetelescope/RegressionTests/actions/runs/11897984206
JWST regtests: https://github.com/spacetelescope/RegressionTests/actions/runs/11897987058
all pass with no AsdfWarning or AsdfDeprecationWarnings.

Tasks

• run pre-commit 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]

The weldx downstream failure is expected.
BAMWelDX/weldx#910 addresses the issues
alternatively BAMWelDX/weldx#952 pins "asdf < 4"

EDIT: weldx merged BAMWelDX/weldx#952

[zacharyburnett]

what does this do?

[braingram]

Thanks! It's an attempt to fix #1844 where towncrier build --keep fails to build for a tagged commit.
For the 4.0.0 release the addition of the 4.0.0 tag will trigger readthedocs to build docs for that version (similar to https://asdf.readthedocs.io/en/3.5.0/). With the last release (the 3.5.0) that one failed because the 3.5.0 tag contained a changelog section for 3.5.0 already and towncrier saw the current commit was tagged so it failed when it tried to add a second 3.5.0 section.
With this command git describe --exact-match || towncrier build --keep the hope is that readthedocs when building the 4.0.0 docs will run the command and since git describe --exact-match will succeed (since the commit is an exact match for a tag) the second part of the command won't be run. This means towncrier build --keep won't run for a tagged commit but I think that's fine since tagged commits are either:

• a commit for a release which will already have a changelog section for that version
• a tag added to main to trigger scm_version to use a newer version where temporarily (for only that tagged commit) the 'latest' docs won't contain the development changelog (but the next commit added to main after the tag will trigger a 'latest' with the changelog)

[braingram]

I pushed a small change to update the 4.0.0 changelog date to today: d081c54

I'm going to merge this then start the release. If all goes well it will go out today. If not I'll update the changelog on main.