It creates a modified copy of an object by calling the object's
__replace__() method.
It is a generalization of dataclasses.replace(), named tuple's _replace()
method and replace() methods in various classes, and supports all these
stdlib classes.
This breaks the tests, but we are keeping it as a separate commit so
that the move operation and editing of the moved files are separate, for
a cleaner history.
Mikhail Samylov
Serhiy Storchaka
William Andrea
Paul Ganssle