input: Write a python function to get the next day from a given date string.
  assert next_day("2020-06-20") == "2020-06-21"
  assert next_day("2021-12-31") == "2022-01-01"
  assert next_day("1999-01-01") == "1999-01-02"
output: def next_day(date_str: str) -> str:
      from datetime import datetime, timedelta
      
      date_format = "%Y-%m-%d"
      date = datetime.strptime(date_str, date_format)
      next_date = date + timedelta(days=1)
      
      return next_date.strftime(date_format)