input: Write a python function to calculate the number of days between two dates.
  assert days_between(datetime.date(2020, 1, 1), datetime.date(2020, 1, 5)) == 4
  assert days_between(datetime.date(2021, 6, 1), datetime.date(2021, 6, 10)) == 9
  assert days_between(datetime.date(2019, 12, 31), datetime.date(2020, 1, 1)) == 1
output: import datetime
  
  def days_between(date1, date2):
      delta = date2 - date1
      return delta.days