perf: all instances that are None are immutable and static

Author: maartenbreddels

traitlets/traitlets.py

@@ -1060,7 +1060,7 @@ def setup_class(cls, classdict):  # noqa
                     elif type(trait) == Union and trait.default_value is None:
                         cls._static_immutable_initial_values[name] = None
                     elif (
-                        type(trait) == Dict
+                        isinstance(trait, Instance)
                         and trait.default_args is None
                         and trait.default_kwargs is None
                         and trait.allow_none