python3Packages.pystemd: fix build

Regressed in cdf2296ce4, which landed in
https://github.com/NixOS/nixpkgs/pull/482364.
Fixes #496811.

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

@@ -50,6 +50,7 @@ buildPythonPackage rec {
 
   disabledTestPaths = [
     "test_version.py" # Requires cstq which is not in nixpkgs
+    "test_pickle.py" # fails with "Could not open a bus to DBus"
   ];
 
   pythonImportsCheck = [ "pystemd" ];