input: Write a function to convert HTML to plain text without using any libraries. The function should take an HTML string as input and return a plain text string with all HTML tags removed.
  assert html_to_text("<h1>Hello, World!</h1>")=="Hello, World!"
  assert html_to_text("<p>This is a paragraph with <b>bold</b> text.</p>")=="This is a paragraph with bold text."
  assert html_to_text("<a href='https://example.com'>Link</a>")=="Link"
  assert html_to_text("No HTML here")=="No HTML here"
  assert html_to_text("")==""
output: def html_to_text(html):
      return ''.join(filter(lambda x: x not in '<>', html)).replace('  ', ' ')