input: Write a python function to remove punctuation and spaces from a string and convert it to lowercase.
  assert clean_string("This i$n't a siMPle StrinG") == "thisisntasimplestring"
  assert clean_string("Hello, World!") == "helloworld"
  assert clean_string("Python's #1!") == "pythons1"
output: def clean_string(s):
      import string
      return ''.join(c for c in s if c.isalnum()).lower()