Now the special comparison methods like `__eq__` and `__lt__` return
NotImplemented if one of comparands is date and other is datetime
instead of ignoring the time part and the time zone or forcefully
return "not equal" or raise TypeError.
It makes comparison of date and datetime subclasses more symmetric
and allows to change the default behavior by overriding
the special comparison methods in subclasses.
It is now the same as if date and datetime was independent classes.
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.
