Override Python interpreter to obtain older version (3.7.2). Just `nix-build -A python372` to build it.

default.nix

let
  overlay = self: super: {
    python372 = let
      py = super.python37.override {
        self = py;
        sourceVersion = {
          major = "3";
          minor = "7";
          patch = "2";
          suffix = "";
        };
        sha256 = "1fzi9d2gibh0wzwidyckzbywsxcsbckgsl05ryxlifxia77fhgyq";
      };
    in py;
  };
in import (fetchTarball "channel:nixos-19.09") {
  overlays = [ overlay ];
}