Trac #34811: rational maps for WeierstrassIsomorphism should not be polynomials

The methods `.rational_maps()` and `.x_rational_map()` for
`WeierstrassIsomorphism` return polynomials, but the same methods for
other `EllipticCurveHom` return rational maps (i.e., elements of the
fraction field).

For consistency, these methods should return the same types in all child
classes of `EllipticCurveHom`, despite the rational maps actually having
denominator `1` in this case.

URL: https://trac.sagemath.org/34811
Reported by: lorenz
Ticket author(s): Lorenz Panny
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/sage/schemes/elliptic_curves/weierstrass_morphism.py

@@ -487,6 +487,8 @@ def __init__(self, E=None, urst=None, F=None):
 
         self._mpoly_ring = PolynomialRing(base_ring, ['x','y'])
         self._poly_ring = PolynomialRing(base_ring, ['x'])
+        self._xyfield = self._mpoly_ring.fraction_field()
+        self._xfield = self._poly_ring.fraction_field()
 
         self._domain = E
         self._codomain = F
@@ -748,14 +750,16 @@ def rational_maps(self):
             sage: w(P).xy() == (f(P.xy()), g(P.xy()))
             True
 
-        TESTS::
+        TESTS:
+
+        Check for :trac:`34811`::
 
             sage: iso.rational_maps()[0].parent()
-            Multivariate Polynomial Ring in x, y over Rational Field
+            Fraction Field of Multivariate Polynomial Ring in x, y over Rational Field
             sage: iso.rational_maps()[1].parent()
-            Multivariate Polynomial Ring in x, y over Rational Field
+            Fraction Field of Multivariate Polynomial Ring in x, y over Rational Field
         """
-        return tuple(baseWI.__call__(self, self._mpoly_ring.gens()))
+        return tuple(baseWI.__call__(self, self._xyfield.gens()))
 
     def x_rational_map(self):
         """
@@ -775,12 +779,14 @@ def x_rational_map(self):
             sage: iso.x_rational_map() == iso.rational_maps()[0]
             True
 
-        TESTS::
+        TESTS:
+
+        Check for :trac:`34811`::
 
             sage: iso.x_rational_map().parent()
-            Univariate Polynomial Ring in x over Rational Field
+            Fraction Field of Univariate Polynomial Ring in x over Rational Field
         """
-        x, = self._poly_ring.gens()
+        x, = self._xfield.gens()
         return (x - self.r) / self.u**2
 
     def kernel_polynomial(self):