gh-36638: return Weierstrass scaling factor in base field for EllipticCurveIsogeny
In the absence of pre- and post-isomorphisms, this value was returned as
an `Integer` instead of in the base field, which is unexpected and
sometimes causes things to break spontaneously.
URL: https://github.com/sagemath/sage/pull/36638
Reported by: Lorenz Panny
Reviewer(s):
