next_sunday with dateutil

from datetime import date, timedelta

from dateutil.rrule import SU, WEEKLY, rrule


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

    if today.weekday() == SU.weekday:
        today += timedelta(days=1)

    return rrule(freq=WEEKLY, dtstart=today, byweekday=SU)[0]