pythonPackages.httpx: Fix tests with sandbox on darwin

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

@@ -56,6 +56,8 @@ buildPythonPackage rec {
     "test_sync_proxy_close"
   ];
 
+  __darwinAllowLocalNetworking = true;
+
   meta = with lib; {
     description = "The next generation HTTP client";
     homepage = "https://github.com/encode/httpx";