diffoscope: fix nixpkgs patch to be black-compliant
diffoscope: fix nixpkgs patch to be black-compliant

Otherwise diffocsope's tests fail as:

    diffoscope> -        LINKS_RE = re.compile(r'Links: [0-9]+\s+')
    diffoscope> +        LINKS_RE = re.compile(r"Links: [0-9]+\s+")
    diffoscope> FAILED tests/test_source.py::test_code_is_black_clean

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