[3.14] gh-102555: Fix comment parsing in HTMLParser according to the HTML5 standard (GH-135664) (GH-136271)

* "--!>" now ends the comment.
* "-- >" no longer ends the comment.
* Support abnormally ended empty comments "<-->" and "<--->".

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(cherry picked from commit 8ac7613dc8b8f82253d7c0e2b6ef6ed703a0a1ee)

Co-author: Kerim Kabirov <the.privat33r+gh@pm.me>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>