A Subclass of python's GenericAlias used in defining and instantiating
Generics.
This class differs from `types.GenericAlias` in that it calls a method
named _parseTypingArgs defined on Fields. This method gives Field and its
subclasses an opportunity to transform type parameters into class key word
arguments. Code authors do not need to implement any returns of this object
directly, and instead only need implement _parseTypingArgs, if a Field
subclass differs from the base class implementation.
This class is intended to be an implementation detail, returned from a
Field's `__class_getitem__` method.