input: Write a function to remove excessive whitespaces from a string.
  assert remove_whitespaces("This     string  contains   too     much whitespace!") == "This string contains too much whitespace!"
  assert remove_whitespaces("  Leading and trailing     spaces should   be removed  ") == "Leading and trailing spaces should be removed"
  assert remove_whitespaces("No     excessive  whitespaces  here") == "No excessive whitespaces here"
output: def remove_whitespaces(s):
      return ' '.join(s.split())