Fix spec generation for traitlets 5

Author: vidartf

packages/schema/generate-spec.py

@@ -114,6 +114,11 @@ def jsdefault(trait):
         default = trait.make_dynamic_default()
         if issubclass(trait.klass, widgets.Widget):
             return 'reference to new instance'
+    elif hasattr(trait, 'default'):
+        # This is the default for traitlets 5
+        default = trait.default()
+        if isinstance(default, bytes) or isinstance(default, memoryview):
+            default = trait.default_value_repr()
     elif isinstance(trait, Union):
         default = trait.make_dynamic_default()
     else: