python.pkgs.trustme: fix python2 build

pkgs/development/python-modules/trustme/default.nix

···
-{ lib, buildPythonPackage, fetchPypi, cryptography, pytest, pyopenssl, service-identity }:
+{ lib, buildPythonPackage, fetchPypi, isPy3k, cryptography, futures, pytest, pyopenssl, service-identity }:
 
 buildPythonPackage rec {
   pname = "trustme";
···
   '';
   propagatedBuildInputs = [
     cryptography
+  ] ++ lib.optionals (!isPy3k) [
+    futures
   ];
 
   meta = {