This is mainly to reduce the amount of code duplication in sage.schemes.elliptic_curves.weierstrass_morphism.isomorphism by turning it into a generator, plus generic style tweaks.

I consider the second commit (renaming isomorphisms() to _isomorphisms()) optional: The function has been marked as "not intended for users" forever, so I figured we might as well just hide it. Note that this does not affect the EllipticCurve_generic.isomorphisms() method in any way.

CC: @JohnCremona

Component: elliptic curves

Author: Lorenz Panny

Branch/Commit: 99a6e3b

Reviewer: John Cremona

Issue created by migration from https://trac.sagemath.org/ticket/34722