input: Write a function to clean a string of HTML tags. The function should return the string without any HTML tags.
  def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("")==""
  assert clean_html_tags("<p>Hello, world!</p>")=="Hello, world!"
  assert clean_html_tags("<div><p>Nested</p><span>tags</span></div>")=="Nestedtags"
  assert clean_html_tags("No tags here")=="No tags here"
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str