Manifest typing and reactive programming for Python (Python 3)

 The traits package provides a metaclass with special attributes that
 are called traits. A trait is a type definition that can be used for
 normal Python object attributes, giving the attributes some
 additional characteristics:
  * Initialization: A trait attribute can have a default value
  * Validation: A trait attribute is manifestly typed.
  * Delegation: The value of a trait attribute can be contained in another
    object
  * Notification: Setting the value of a trait attribute can fired
    callbacks
  * Visualization: With the TraitsUI package, GUIs can be generated
    automatically from traited objects.
 .
 This is the Python 3 version of the package.