input: Write a function to get the day name from a given date string in the format 'YYYY-MM-DD'.
  assert get_day_name('2021-04-19') == 'Monday'
  assert get_day_name('2022-12-25') == 'Sunday'
  assert get_day_name('2020-01-01') == 'Wednesday'
output: def get_day_name(date_string):
      from datetime import datetime
      date_object = datetime.strptime(date_string, '%Y-%m-%d')
      return date_object.strftime('%A')