reprod:

from astropy.io.fits.util import itersubclasses
list(itersubclasses(object))

Traceback (most recent call last):
  File "<python-input-1>", line 2, in <module>
    list(itersubclasses(object))
    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "<...>/.venv/lib/python3.14/site-packages/astropy/io/fits/util.py", line 187, in itersubclasses
    if sub not in _seen:
       ^^^^^^^^^^^^^^^^
TypeError: cannot use 'typing._UnionGenericAliasMeta' as a set element (unhashable type: '_UnionGenericAliasMeta')

typing._UnionGenericAliasMeta is a private implementation detail from the standard library. It's furthermore mentioned on the What's New page as follow

Previously, old-style unions were implemented using the private class typing._UnionGenericAlias. This class is no longer needed for the implementation, but it has been retained for backward compatibility, with removal scheduled for Python 3.17. Users should use documented introspection helpers like typing.get_origin() and typing.get_args() instead of relying on private implementation details

We could probably just special-case it as a last resort, but it's still pretty surprising that the class itself became unhashable all of a sudden, so I'll also report upstream so CPython folks have a chance of fixing this in time for the final release if they think it's appropriate.