make the shell.nix environment support elpy

requirements/elpy.in

+# support elpy
+pip
+setuptools
+jedi

shell.nix

@@ -18,6 +18,7 @@ let
     requirements =
       ''
       ${builtins.readFile ./requirements/test.in}
+      ${builtins.readFile ./requirements/elpy.in}
       ${zkapauthorizer.requirements}
       '';
   };