[3.13] gh-135462: Fix quadratic complexity in processing special input in HTMLParser (GH-135464) (GH-135482)

End-of-file errors are now handled according to the HTML5 specs --
comments and declarations are automatically closed, tags are ignored.
(cherry picked from commit 6eb6c5dbfb528bd07d77b60fd71fd05d81d45c41)

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