use new pytest asdf plugin

[braingram]

Update test dependencies to use new https://pypi.org/project/pytest-asdf-plugin/ which is a feature-compatible replacement for the bundled asdf pytest plugin (which will soon be deprecated).

For context the pytest-asdf-plugin is responsible for finding and converting schema files into "tests" (to check the schema against the metaschema and validated any contained "examples"). Since both the bundled and now external plugins are feature-compatible there should be no change in test number (or new failures, etc).

On main 572 tests passed:
https://github.com/astropy/specutils/actions/runs/16924368657/job/47957149583#step:5:183

With this PR 572 tests passed:
https://github.com/astropy/specutils/actions/runs/17070847734/job/48399089755?pr=1265#step:5:184

Let me know if this needs a changelog if if there is a label to add to skip this check (I don't have permissions to change the labels).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.84%. Comparing base (ef3d727) to head (c7a1131).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1265   +/-   ##
=======================================
  Coverage   86.84%   86.84%           
=======================================
  Files          63       63           
  Lines        4743     4743           
=======================================
  Hits         4119     4119           
  Misses        624      624           

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[rosteen]

Thanks, this wasn't on my radar at all.