input: Write a python function to clean a string of HTML tags.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>") == "This is a test string"
  assert clean_html_tags("<div>Hello <span>world!</span></div>") == "Hello world!"
  assert clean_html_tags("<h1>Header</h1><p>Paragraph</p>") == "HeaderParagraph"
output: def clean_html_tags(html_string):
      import re
      clean_text = re.sub('<.*?>', '', html_string)
      return clean_text