A shell I can run the tests in

don't like the duplication but not sure how to avoid it

shell.nix

 { pkgs ? import <nixpkgs> { } }:
+let
+  satauthorizer = pkgs.callPackage ./default.nix { };
+in
   (pkgs.python27.buildEnv.override {
     extraLibs = [
-    (pkgs.callPackage ./default.nix { })
+      pkgs.python27Packages.fixtures
+      pkgs.python27Packages.testtools
+      pkgs.python27Packages.hypothesis
+      satauthorizer
     ];
     ignoreCollisions = true;
   }).env