Eliminate evolve() boilerplate with evolvers/copy

When I want to evolve() an object, I sometimes have to do a bit of boilerplate copy()ing:

from typing import List

import attr


@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib()


car1 = Car("Toyota", ["Alice", "Bob"])
car2 = attr.evolve(car1, model="Honda", occupants=car1.occupants.copy())

To eliminate this boilerplate, one option would be:

@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib(evolver=lambda x: x.copy())

Or similarly:

@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib()

    @occupants
    def evolver(self):
        return self.occupants.copy()

Or using copy.copy() via the __copy__ method:

@attr.s
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib(evolve_copy=True)
    

The latter idea suggests the option of putting the copy functionality into @attr.s, which could add a __copy__ method to the class, and like the other methods it could be enabled/disabled for each attribute with attr.ib(copy=True/False), and evolve() could use copy.copy() by default.

@attr.s(copy=True)
class Car:
    model: str = attr.ib()
    occupants: List = attr.ib()
    components: pyrsistent.PSet = attr.ib(copy=False)

cc: @altendky

Start by reviewing the existing evolve(), attr.s, and attr.ib behavior described in the issue, then compare the proposed evolver, decorator, and copy approaches. The issue does not name implementation files or tests; the work is done when one agreed API is specified, implemented, and covered by tests for copied and non-copied attributes.