input: Write a function to calculate days since a given date in YYYY-MM-DD format.
  assert days_since("2023-01-01") == (date.today() - date(2023, 1, 1)).days
  assert days_since("2020-05-15") == (date.today() - date(2020, 5, 15)).days
  assert days_since("2022-12-31") == (date.today() - date(2022, 12, 31)).days
output: from datetime import date
  
  def days_since(given_date):
      given_date = date.fromisoformat(given_date)
      return (date.today() - given_date).days