input: Write a python function to remove special characters from a string.
  assert remove_special_characters('Hello  World!!!') == 'HelloWorld'
  assert remove_special_characters('Python@3.9') == 'Python39'
  assert remove_special_characters('123#456*789') == '123456789'
output: def remove_special_characters(s):
      return ''.join(e for e in s if e.isalnum())