Make umu_delta optional.
--- a/umu/umu_proton.py
+++ b/umu/umu_proton.py
@@ -524,7 +524,10 @@ def _get_delta(
     from cbor2 import CBORDecodeError, dumps, loads  # noqa: PLC0415
 
     # Ignore. umu_delta is relevant for sys packages when using *-Latest tokens
-    from .umu_delta import valid_key, valid_signature  # noqa: PLC0415
+    try:
+        from .umu_delta import valid_key, valid_signature  # noqa: PLC0415
+    except ImportError:
+        return None
 
     try:
         cbor = loads(patch)