get rid of python2 mentioned in .github/workflows and README.md

[dimpase]

observed in #22191#comment:244

also, we should not mention python2 as an example in README.md

CC: @mkoeppe

Component: misc

Author: Dima Pasechnik

Branch/Commit: 8195cb8

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30048

[dimpase]

Branch: u/dimpase/misc/nopy2

[sagetrac-git]

Commit: 8195cb8

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

8195cb8

no python2!

[dimpase]

Author: Dima Pasechnik

[mkoeppe]

Reviewer: Matthias Koeppe

[vbraun]

Changed branch from u/dimpase/misc/nopy2 to 8195cb8