aria2: refactor to avoid python2 dependency
aria2: refactor to avoid python2 dependency

Use python3 Sphinx package instead of python2 Sphinx package,
to avoid build-time dependency on python 2, as per
https://github.com/NixOS/nixpkgs/issues/101964

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs