RFC: Allow predicate value for `status_forcelist`

[mharrisb1]

Request for comment (RFC) on potential feature:

Allow predicate with signature Callable[[int], bool] to be used for status_forcelist.

This can allow a user to add range of status codes without explicitly enumerating them.

Example:

from httpx_retries import Retry

def is_5XX_status(code: int) -> bool:
    return code >= 500

rules = Retry(status_forcelist=is_5XX_status)

[will-ockmore]

Hey @mharrisb1 , this is a good addition. We should still allow the existing list of status codes, but also allowing a predicate gives more flexibility.