Bundle slots into a list if use_attributes is True

In this way, the slot definition in `slots` can be added
to the class as an attribute, better than raising an
exception

linkml_runtime/utils/schema_builder.py

@@ -82,9 +82,7 @@ def add_class(
         self.schema.classes[cls.name] = cls
         if use_attributes:
             if not isinstance(slots, list):
-                raise ValueError(
-                    "If `use_attributes=True`, then `slots` must be a list."
-                )
+                slots = [slots]
             for s in slots:
                 if isinstance(s, SlotDefinition):
                     cls.attributes[s.name] = s