GaloisGroup_subgroup.fixed_field() should always return an embedding

[pbruin]

Currently, GaloisGroup_subgroup.fixed_field() does not return an embedding of the fixed field into the ambient field if the subgroup is trivial:

sage: K.<a> = NumberField(x^6 - 6*x^4 + 9*x^2 + 23)
sage: G = K.galois_group()
sage: H = G.subgroup([g for g in G if g.order().divides(3)])
sage: H.fixed_field()
(Number Field in a0 with defining polynomial x^2 + 207, Ring morphism:
   From: Number Field in a0 with defining polynomial x^2 + 207
   To:   Number Field in a with defining polynomial x^6 - 6*x^4 + 9*x^2 + 23
   Defn: a0 |--> 3*a^3 - 9*a)
sage: H = G.subgroup([G.identity()])
sage: H.fixed_field()
Number Field in a with defining polynomial x^6 - 6*x^4 + 9*x^2 + 23

This should be fixed for consistency.

[pbruin]

This branch fixes the problem by removing two lines that "work around a silly error". David, you seem to be the author of this code; do you remember what this was? Maybe a bug in an old PARI version?

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​79e2a58	Merge branch 'develop' into ticket/26817-GaloisGroup_fixed_field
​2b26bc4	Trac 26817: fix doctest

[roed]

Looks good to me. The recent test failures are due to the switch to Python 3 breaking patchbots.