input: Write a python function to find the day of the week for a given date string in "YYYY-MM-DD" format.
  assert day_of_week("2023-10-05") == "Thursday"
  assert day_of_week("2020-02-29") == "Saturday"
  assert day_of_week("2000-01-01") == "Saturday"
output: def day_of_week(date_string):
      import datetime
      days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
      date_obj = datetime.datetime.strptime(date_string, "%Y-%m-%d")
      return days[date_obj.weekday()]