app: Fix build issue with Django 5.x

Closes: #1131272 (Debian)

Tests:

- With Django 4.2 (stable container), syncthing app installs properly and home
page shows the tags properly.

- With Django 6.0 (unstable+experimental container), syncthing app installs
properly and home page shows the tags properly.

- With a cowbuilder base image of Ubuntu resolute distribution, freedombox
package builds successfully with gbp.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Joseph Nuthalapati <njoseph@riseup.net>

plinth/app.py

@@ -574,10 +574,19 @@ class Info(FollowerComponent):
         except ImproperlyConfigured:
             # Hack to allow apps to be instantiated without Django
             # initialization as required by privileged process.
-            return [
-                tag._proxy____args[0] if isinstance(tag, Promise) else tag
-                for tag in self._tags
-            ]
+            def _make_str(tag):
+                """Return the string without casting."""
+                if not isinstance(tag, Promise):
+                    return tag
+
+                # Django 4.2
+                if hasattr(tag, '_proxy____args'):
+                    return tag._proxy____args[0]
+
+                # Django 5.x
+                return tag._args[0]
+
+            return [_make_str(tag) for tag in self._tags]
 
 
 class EnableState(LeaderComponent):