isso: fix tests

Due to some dependency bump, isso ceased to build its tests for a deprecated API usage.
A PR has been sent upstream, see in the diff and it is applied via substitution here.

pkgs/servers/isso/default.nix

@@ -23,6 +23,12 @@ buildPythonApplication rec {
     "doc"
   ];
 
+  postPatch = ''
+    # Remove when https://github.com/posativ/isso/pull/973 is available.
+    substituteInPlace isso/tests/test_comments.py \
+      --replace "self.client.delete_cookie('localhost.local', '1')" "self.client.delete_cookie(key='1', domain='localhost')"
+  '';
+
   propagatedBuildInputs = [
     itsdangerous
     jinja2