pythonPackages.webtest: fix darwin build

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

@@ -30,6 +30,9 @@ buildPythonPackage rec {
 
   checkInputs = [ nose mock PasteDeploy wsgiproxy2 pyquery ];
 
+  # Some of the tests use localhost networking.
+  __darwinAllowLocalNetworking = true;
+
   meta = with stdenv.lib; {
     description = "Helper to test WSGI applications";
     homepage = https://webtest.readthedocs.org/en/latest/;