nixosTests.freshrss.extensions: replace lxml-stubs with types-lxml

lxml-stubs is broken, unmaintained upstream, and advertises types-lxml
as its successor [1].

Also don't skip type checking anymore as it now passes.

[1] https://github.com/lxml/lxml-stubs

nixos/tests/freshrss/extensions.nix

@@ -16,9 +16,8 @@
     };
   extraPythonPackages = p: [
     p.lxml
-    p.lxml-stubs
+    p.types-lxml
   ];
-  skipTypeCheck = true;
 
   testScript = ''
     machine.wait_for_unit("multi-user.target")