pkgSource = https://github.com/nixos/nixpkgs/archive/552d8a70d4c47a6e52a8e0b26e3a2e33d5b94ca6.tar.gz;
pkgTarball = fetchTarball pkgSource;
pkgs = import pkgTarball {};
my-python = pkgs.python37.withPackages(ps: with ps; [ virtualenv ]);