input: Write a python function to check if a string contains only digits.
  assert is_digit_only("0123") == True
  assert is_digit_only("123abc") == False
  assert is_digit_only("007") == True
  assert is_digit_only("") == True
  assert is_digit_only(" ") == False
output: def is_digit_only(s):
      return s.isdigit()