input: Write a python function to validate a date in the format 'dd/mm/yyyy'.
  assert validate_date('31/12/2020') == True
  assert validate_date('31/02/2020') == False
  assert validate_date('29/02/2020') == True
output: def validate_date(date_str):
      from datetime import datetime
      try:
          datetime.strptime(date_str, '%d/%m/%Y')
          return True
      except ValueError:
          return False