Fix bug in covering map of simplicial set with degenerate faces.

[miguelmarco]

There is a bug in the method to compute universal covers of simplicial sets. It is triggered when a nondegenerate cell
has degenerate faces:

sage: RP2 = simplicial_sets.RealProjectiveSpace(2)
sage: S3 = simplicial_sets.Sphere(3)
sage: X = S3.wedge(RP2)
sage: XU = X.universal_cover()
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[5], line 1
----> 1 X.universal_cover()

File /usr/local/sage97/src/sage/categories/simplicial_sets.py:581, in SimplicialSets.Pointed.ParentMethods.universal_cover(self)
    557 def universal_cover(self):
    558     r"""
    559     Return the universal cover of the simplicial set.
    560     The fundamental group must be finite in order to ensure that the
   (...)
    579         Finitely presented group <  |  >
    580     """
--> 581     return self.universal_cover_map().domain()

File /usr/local/sage97/src/sage/categories/simplicial_sets.py:429, in SimplicialSets.Pointed.ParentMethods.universal_cover_map(self)
    427     return self.identity()
    428 G, char = self._universal_cover_dict()
--> 429 return self.covering_map(char)

File /usr/local/sage97/src/sage/categories/simplicial_sets.py:499, in SimplicialSets.Pointed.ParentMethods.covering_map(self, character)
    497             char[s] = char[s.nondegenerate()]
    498     else:
--> 499         char[s] = char[self.face(s, d)]
    500 if s.is_nondegenerate():
    501     for g in G:

KeyError: s_1 s_0 *

This PR fixes this.

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[jhpalmieri]

Looks good, thanks for catching fixing this.