[3.13] gh-149489: Fix ElementTree serialization to HTML (GH-149490) (GH-150596) ...

[3.13] gh-149489: Fix ElementTree serialization to HTML (GH-149490) (GH-150596) (GH-150609)

* The content of elements "xmp", "iframe", "noembed", "noframes",
  and "plaintext" is no longer escaped.
* The "plaintext" element no longer have the closing tag.
(cherry picked from commit c42e6d3f1a066186f74d3971df4c512bc11c7997)

(cherry picked from commit bcd29e466f55d8b4e3849ed6ada8ce86a46f5072)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>