ci: fix xandikos test server

Newer xandikos versions renamed XandikosBackend to
SingleUserFilesystemBackend.  Use a try/except fallback matching
the approach already used in the caldav library's testing.py.

prompt: The github checks failed on PR33, please investigate. Note that
another branch is currently checked out, so either wait editing any files
or create a separate worktree. [follow-up] The tests fail locally also.
The code for setting up the xandikos test server does not work with the
latest xandikos versions. Check ~/caldav on how things are done there.
[follow-up] This fix should probably go in as a separate pull request,
forked off the main branch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tobixen

tests/test_functional.py

@@ -14,7 +14,12 @@
 import aiohttp
 import aiohttp.web
 import niquests as requests
-from xandikos.web import XandikosApp, XandikosBackend
+from xandikos.web import XandikosApp
+
+try:
+    from xandikos.web import SingleUserFilesystemBackend as XandikosBackend
+except ImportError:
+    from xandikos.web import XandikosBackend  # type: ignore[no-redef]
 
 from plann.cli import _add_journal, _add_todo, _check_for_panic, _list, _select
 from plann.interactive import (