gh-38944: no longer ignore errors in method `union` of `DisjointSet`
    
Fixes #38939.

This PR changes the declaration of method `union` from `noexcept` to
`except *` to properly raise errors.

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- [x] The description explains in detail what this PR is about.
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preview.

### ⌛ Dependencies

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URL: #38944
Reported by: David Coudert
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/sets/disjoint_set.pxd

@@ -29,7 +29,7 @@ cdef class DisjointSet_of_hashables(DisjointSet_class):
     cdef list _int_to_el
     cdef dict _el_to_int
     cpdef find(self, e)
-    cpdef void union(self, e, f) noexcept
+    cpdef void union(self, e, f) except *
     cpdef root_to_elements_dict(self)
     cpdef element_to_root_dict(self)
     cpdef to_digraph(self)

src/sage/sets/disjoint_set.pyx

@@ -833,7 +833,7 @@ cdef class DisjointSet_of_hashables(DisjointSet_class):
         cdef int r = <int> OP_find(self._nodes, i)
         return self._int_to_el[r]
 
-    cpdef void union(self, e, f) noexcept:
+    cpdef void union(self, e, f) except *:
         r"""
         Combine the set of ``e`` and the set of ``f`` into one.
 
@@ -861,8 +861,9 @@ cdef class DisjointSet_of_hashables(DisjointSet_class):
             sage: e
             {{'a', 'b', 'c', 'e'}, {'d'}}
             sage: e.union('a', 2**10)
-            KeyError: 1024
+            Traceback (most recent call last):
             ...
+            KeyError: 1024
         """
         cdef int i = <int> self._el_to_int[e]
         cdef int j = <int> self._el_to_int[f]