next_sunday without dateutil

from datetime import date, timedelta


def next_sunday(today: date | None = None) -> date:
    if today is None:
        today = date.today()

    one_day = timedelta(days=1)

    if today.weekday() == 6:
        today += one_day

    while today.weekday() != 6:
        today += one_day

    return today