[3.15] gh-149489: Fix ElementTree serialization to HTML (GH-149490) (GH-150595)

[3.15] gh-149489: Fix ElementTree serialization to HTML (GH-149490) (GH-150595)

* The content of comments, processing instructions and elements "xmp",
  "iframe", "noembed", "noframes", and "plaintext" is no longer escaped.
* The "plaintext" element no longer have the closing tag.
* Add support of empty attributes (with value None).
(cherry picked from commit bcd29e466f55d8b4e3849ed6ada8ce86a46f5072)