python.pkgs.backports_lzma: set to null conditionally

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

···
 , isPy3k
 , lzma
 , python
+, pythonOlder
 }:
 
-buildPythonPackage rec {
+if !(pythonOlder "3.3") then null else buildPythonPackage rec {
   pname = "backports.lzma";
   version = "0.0.9";