Add a check and if python does not exist but python3 does, use that.
If neither is present, error out.

This is to ease the migration to python3 (see https://peps.python.org/pep-0394/)

See https://peps.python.org/pep-0237/. Long and int are the same length
on python3 (and long does not exist there any longer)

- Use four spaces instead of tabs
- Use 'is' for tests against None
- Run the whole file through the black linter

The recently released CMake 4 drops support below CMake 3.5, causing a
build failure if the minimum required version is specified below 3.5.

This commit just bumps the minimum version; project still builds fine.

Reported in Debian in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1113165

Bump CMake minimum version to 3.5

Cleanups for python3

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/0196c0175e9191c474c26ab5548db27ef5d34b05?narHash=sha256-WGaHVAjcrv%2BCun7zPlI41SerRtfknGQap281%2BAakSAw%3D' (2025-02-24)
  → 'github:nixos/nixpkgs/0b4defa2584313f3b781240b29d61f6f9f7e0df3?narHash=sha256-Oncbh0UmHjSlxO7ErQDM3KM0A5/Znfofj2BSzlHLeVw%3D' (2025-10-09)