sagemathgh-36040: Fix failing doctest in sage/graphs/bliss.pyx
    
Since 10.1.beta9, a doctest is failing in `sage/graphs/bliss.pyx` due to
some changes in `sage/groups/perm_gps/permgroup.py`. One of them is that
method `.gens()` now returns a tuple and no longer a list.

```py
        sage: # optional - bliss
        sage: from sage.graphs.bliss import automorphism_group
        sage: alpha = "abcdefghijklmnopqrstuvwxyz"
        sage: G = Graph()
        sage: G.add_edges((alpha[i],alpha[j],"A") for i in range(0, 2)
for j in range(14,20))
        sage: G.add_edges((alpha[i],alpha[j],"B") for i in range(2, 5)
for j in range(14,20))
        sage: G.add_edges((alpha[i],alpha[j],"C") for i in range(5, 9)
for j in range(14,20))
        sage: G.add_edges((alpha[i],alpha[j],"D") for i in range(9,14)
for j in range(14,20))
        sage: A = automorphism_group(G)
        sage: print(A.gens())
```

```py
Failed example:
    print(A.gens())
Expected:
    [('r','t'), ('s','r'), ('p','s'), ('q','p'), ('o','q'), ('l','n'),
     ('m','l'), ('j','m'), ('k','j'), ('i','h'), ('f','i'), ('g','f'),
     ('e','d'), ('c','e'), ('a','b')]
Got:
    (('m','n'), ('l','m'), ('k','l'), ('j','k'), ('h','i'), ('g','h'),
('f','g'), ('d','e'), ('c','d'), ('s','t'), ('r','s'), ('q','r'),
('p','q'), ('o','p'), ('a','b'))
```


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URL: sagemath#36040
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton