Fix conf-python-3 on Centos

[jmid]

Spotted on #28750:
https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/0fd2245f35e2c69f0d17ec627e7c379c755893f8/variant/distributions,centos-10-ocaml-5.4,unikraft.0.20.0

+ /usr/bin/sudo "yum" "install" "-y" "python39"
- Extra Packages for Enterprise Linux 10 - x86_64  15 MB/s | 5.7 MB     00:00    
- Last metadata expiration check: 0:00:02 ago on Tue Oct 21 13:16:38 2025.
- No match for argument: python39
- Error: Unable to find a match: python39
[ERROR] System package install failed with exit code 1 at command:
            sudo yum install -y python39
[ERROR] These packages are still missing: epel-release python39

AFAICS, Centos 10 offers Python 3.12.

• If I am reading the package name correctly here: https://mirror.stream.centos.org/10-stream/BaseOS/x86_64/os/Packages/ there should a python3 package available.
• https://repology.org/project/python/versions for Centos 10 however lists no python3 package and
• https://gitlab.com/redhat/centos-stream/rpms/python3/-/tree/c10s?ref_type=heads says dead.package 😬

so this is an experiment... 🤞

[jmid]

Yay, that worked! It is green on all distributions, disregarding revdeps failures.
It should thus be good to go.