input: Write a python function to convert a string into its ASCII representation.
  assert ascii_repr_str("Python") == "080121116104111110"
  assert ascii_repr_str("Hello") == "072101108108111"
  assert ascii_repr_str("ABC") == "065066067"
output: def ascii_repr_str(s):
      return ''.join(f'{ord(c):03}' for c in s)