Make umu_delta optional.
--- a/umu/umu_proton.py
+++ b/umu/umu_proton.py
@@ -517,7 +517,10 @@ def _get_delta(
 
     from cbor2 import CBORDecodeError, dumps, loads
 
-    from .umu_delta import valid_key, valid_signature
+    try:
+        from .umu_delta import valid_key, valid_signature
+    except ImportError:
+        return None
 
     try:
         cbor = loads(patch)