zict: disable build for Python older than 3.6

No longer supported as of
https://github.com/dask/zict/commit/4d5e14dadba940235a59f7e5076653cf7d51d27d

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

@@ -1,5 +1,5 @@
 { stdenv, buildPythonPackage, fetchPypi
-, pytest, heapdict }:
+, pytest, heapdict, pythonOlder }:
 
 buildPythonPackage rec {
   pname = "zict";
@@ -10,6 +10,8 @@ buildPythonPackage rec {
     sha256 = "8e2969797627c8a663575c2fc6fcb53a05e37cdb83ee65f341fc6e0c3d0ced16";
   };
 
+  disabled = pythonOlder "3.6";
+
   buildInputs = [ pytest ];
   propagatedBuildInputs = [ heapdict ];