pythonPackages.pythonefl: does not install egg

With the egg, econnman does not find the modules efl and elementary

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

@@ -34,7 +34,7 @@ buildPythonPackage rec {
   '';
 
   installPhase = ''
-    ${python.interpreter} setup.py install --prefix=$out
+    ${python.interpreter} setup.py install --prefix=$out --single-version-externally-managed
   '';
 
   doCheck = false;