Signature objects are *immutable*. Use :meth:`Signature.replace` to make a
modified copy.
+ .. versionchanged:: 3.5
+ Signature objects are picklable.
+
.. attribute:: Signature.empty
A special class-level marker to specify absence of a return annotation.
Parameter objects are *immutable*. Instead of modifying a Parameter object,
you can use :meth:`Parameter.replace` to create a modified copy.
+ .. versionchanged:: 3.5
+ Parameter objects are picklable.
+
.. attribute:: Parameter.empty
A special class-level marker to specify absence of default values and