input: Write a function to remove punctuations and special characters from a string.
  assert clean_string("Hello, World!")=="Hello World"
  assert clean_string("Python@#2023!!")=="Python2023"
  assert clean_string("Clean_this&text#now$")=="Cleanthistextnow"
output: def clean_string(s):
      return ''.join(char for char in s if char.isalnum() or char.isspace())