input: Write a function to clean a string of HTML tags. The function should remove all HTML tags from the given string and return the cleaned string.
  
  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("Hello, <a href='http://example.com'>world</a>!") == "Hello, world!"
  assert clean_html_tags("<div><p>Some text</p></div>") == "Some text"
  assert clean_html_tags("") == ""
  assert clean_html_tags("No tags here") == "No tags here"
output: def clean_html_tags(str):
      clean_str = re.sub("<.*?>", "", str)
      return clean_str