input: Write a python function to remove punctuation from a given string.
  assert clean_string("Hello, World!") == "Hello World"
  assert clean_string("Python's syntax is easy-to-learn.") == "Pythons syntax is easytolearn"
  assert clean_string("Let's test: some, punctuation!") == "Lets test some punctuation"
output: def clean_string(s):
      import string
      return ''.join(c for c in s if c not in string.punctuation)