Add an introduction or exercise for dataclass in Ch. 18?

[alchemistcai]

Since the book already cover namedtuple in Ch.18, maybe consider adding dataclass as well? It's been in the stdlib since 3.7 and is pretty common nowadays.

It'd be a nice complement to namedtuple—mutable, supports methods, less boilerplate than writing a regular class. Readers would find it useful.