Trac #21704: bliss does not support multiple edges

Example (with bliss installed):
{{{#!python
sage: G = Graph(':C_kQ')
sage: G.automorphism_group(algorithm='bliss').order()
8
sage: G.automorphism_group(algorithm='sage').order()
4
}}}

This ticket changes the default behaviour so the `'sage'` algorithm is
used by default when dealing with multiedges, and an error is raised if
`'bliss'` is given explicitly. Similar situation for `canonical_label`.

URL: https://trac.sagemath.org/21704
Reported by: jaanos
Ticket author(s): Jori Mäntysalo
Reviewer(s): Travis Scrimshaw

src/sage/graphs/generic_graph.py

@@ -20111,10 +20111,12 @@ def coarsest_equitable_refinement(self, partition, sparse=True):
     def automorphism_group(self, partition=None, verbosity=0,
                            edge_labels=False, order=False,
                            return_group=True, orbits=False, algorithm=None):
-        """Returns the largest subgroup of the automorphism group of the
-        (di)graph whose orbit partition is finer than the partition given.
-        If no partition is given, the unit partition is used and the entire
-        automorphism group is given.
+        """
+        Return the automorphism group of the graph.
+
+        With ``partition`` this can also return the largest subgroup
+        of the automorphism group of the (di)graph whose orbit
+        partition is finer than the partition given.
 
         INPUT:
 
@@ -20299,14 +20301,19 @@ def automorphism_group(self, partition=None, verbosity=0,
             sage: G.subgroups()
             [Subgroup of (Permutation Group with generators [(0,7)(1,4)(2,3)(6,8)]) generated by [()],
             Subgroup of (Permutation Group with generators [(0,7)(1,4)(2,3)(6,8)]) generated by [(0,7)(1,4)(2,3)(6,8)]]
-
         """
         try:
             from sage.graphs.bliss import automorphism_group
             have_bliss = True
         except ImportError:
             have_bliss = False
 
+        # See trac #21704
+        if self.has_multiple_edges():
+            if algorithm == 'bliss':
+                raise NotImplementedError("algorithm 'bliss' can not be used for graph with multiedges")
+            have_bliss = False
+
         if (algorithm == 'bliss'           or   # explicit choice from the user; or
             (algorithm is None             and  # by default
              not edge_labels               and
@@ -20982,6 +20989,12 @@ def canonical_label(self, partition=None, certificate=False, verbosity=0,
         except ImportError:
             have_bliss = False
 
+        # See trac #21704
+        if self.has_multiple_edges():
+            if algorithm == 'bliss':
+                raise NotImplementedError("algorithm 'bliss' can not be used for graph with multiedges")
+            have_bliss = False
+
         if (algorithm == 'bliss'           or  # explicit request; or
             (algorithm is None             and # default choice
              have_bliss and