input: Write a python function to remove punctuation from a given string.
  assert remove_punctuation('Hello, world!') == 'Hello world'
  assert remove_punctuation('Good morning, Dr. Smith.') == 'Good morning Dr Smith'
  assert remove_punctuation('This is a test...') == 'This is a test'
output: def remove_punctuation(input_string):
      import string
      return ''.join(char for char in input_string if char not in string.punctuation)