performance: Fix failure to start due to lru_cache in stable

Ability to use lru_cache(user_function) was only introduced in Python 3.8.
Buster has Python 3.7. This causes startup failure in Buster. Fix this by using
the older API for lru_cache instead of the new one.

Tests:

- On stable and testing, starting the Plinth daemon and visiting the performance
page should work.

- Adding a print statement in the _get_url() method shows that it is only called
once, ever.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/performance/manifest.py

@@ -12,7 +12,7 @@ from django.utils.translation import ugettext_lazy as _
 from plinth.utils import Version
 
 
-@lru_cache
+@lru_cache()
 def _get_url():
     """Return the web client URL based on Cockpit version."""
     process = subprocess.run(