use CommutativeRing in ring_extension

[fchapoton]

Removing one of the last uses of CommutativeAlgebra class, towards its deprecation.

Also a few pep8 fixes in the modified file.

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[github-actions]

Documentation preview for this PR (built with commit 8de95f8; changes) is ready! 🎉

[mantepse]

Does this actually mean that inheriting from CommutativeAlgebra doesn't do anything beyond making

sage: isinstance(K, CommutativeAlgebra)
True

?

More precisely, I tried:

sage: F = GF(5^2)
sage: k = GF(5^4)
sage: z4 = k.gen()
sage: K.<a> = k.over(F, gen=1-z4); K
Field in a with defining polynomial x^2 + z2*x + 4 over its base
sage: K in Algebras(K.base_ring())
False
sage: from sage.rings.ring import CommutativeAlgebra
sage: isinstance(K, CommutativeAlgebra)
True

and with the new branch, the last line is False (which looks completely OK, of course, I am just surprised).

(I admit that I'm not completely sure what ring_extension.pyx actually does)

[fchapoton]

yes, probably your are right. The usage of isinstance should be avoided and categories should be used instead.

My aim for now is to keep only the bad class CommutativeRing and deprecate several others auld-parent classes like Algebra, CommutativeAlgebra, etc. Then and only then, I will try to disentangle what happens for all commutative rings (and this will be a huge mess)

The change here is a step towards deprecating CommutativeAlgebra, which is almost no longer used already.

[mantepse]

Thank you!