input: Write a function to convert a date string in "DD/MM/YYYY" format to a datetime object.
  assert convert_to_datetime("27/09/2017") == datetime.datetime(2017, 9, 27)
  assert convert_to_datetime("01/01/2020") == datetime.datetime(2020, 1, 1)
  assert convert_to_datetime("15/08/1947") == datetime.datetime(1947, 8, 15)
output: def convert_to_datetime(date_str):
      from datetime import datetime
      return datetime.strptime(date_str, "%d/%m/%Y")