DEV: Convert a stray import_item -> self._resolve_string.

IPython/utils/traitlets.py

@@ -819,7 +819,7 b' class Type(ClassBasedTraitType):'
         """Validates that the value is a valid object instance."""
         if isinstance(value, py3compat.string_types):
             try:
-                value = import_item(value)
+                value = self._resolve_string(value)
             except ImportError:
                 raise TraitError("The '%s' trait of %s instance must be a type, but "
                                 "%r could not be imported" % (self.name, obj, value))