Motivation for this change

This fixes an evaluation error introduced by #108978:

at: (5:19) in file: /home/.../nixpkgs/pkgs/applications/networking/instant-messengers/salut-a-toi/default.nix

     4|   inherit (python27Packages) python;
     5|   requirements = (import ./requirements.nix {
      |                   ^
     6|     inherit stdenv fetchurl;

anonymous function at /home/.../nixpkgs/pkgs/applications/networking/instant-messengers/salut-a-toi/requirements.nix:1:1 called without required argument 'lib'

The package still can't be build because of broken dependencies:

$ nix build -f . salut_a_toi
error: --- ThrownError ----------------------------------------------------------------------- nix
urwid-2.1.2 not supported for interpreter python2.7

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.