pythonPackages.stem: remove marked as broken

A third attempt after #85642 which I hadn't recognized as a backport
of #81679.

The bug is just with the test suite on python 3.8 which was fixed in 1.8.0.
See bug report: https://trac.torproject.org/projects/tor/ticket/30847.

(cherry picked from commit 618cdd2473)

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

@@ -26,6 +26,5 @@ buildPythonPackage rec {
     homepage = https://stem.torproject.org/;
     license = licenses.gpl3;
     maintainers = with maintainers; [ phreedom ];
-    broken = true;
   };
 }