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: #36638
Reported by: Lorenz Panny
Reviewer(s):

Author: Release Manager

src/sage/schemes/elliptic_curves/ell_curve_isogeny.py

@@ -2838,11 +2838,18 @@ def scaling_factor(self):
             sage: phi.dual().scaling_factor()
             43
 
+        TESTS:
+
+        Check for :issue:`36638`::
+
+            sage: phi.scaling_factor().parent()  # needs sage.rings.finite_rings
+            Finite Field in z2 of size 257^2
+
         ALGORITHM: The "inner" isogeny is normalized by construction,
         so we only need to account for the scaling factors of a pre-
         and post-isomorphism.
         """
-        sc = Integer(1)
+        sc = self.__base_field.one()
         if self.__pre_isomorphism is not None:
             sc *= self.__pre_isomorphism.scaling_factor()
         if self.__post_isomorphism is not None: