input: Write a function to clean a string of HTML tags using Python. The function should take a string containing HTML tags as input and return a string with all HTML tags removed.
  assert clean_html_tags("<p>This is a <b>test</b> string</p>")=="This is a test string"
  assert clean_html_tags("<div><h1>Heading</h1><p>Paragraph</p></div>")=="HeadingParagraph"
  assert clean_html_tags("<a href='#'>Link</a>")=="Link"
  assert clean_html_tags("<p>No tags here</p>")=="No tags here"
  assert clean_html_tags("")==""
output: def clean_html_tags(html_string):
      import re
      return re.sub(r'<[^>]+>', '', html_string)