[3.13] gh-135661: Fix parsing unterminated bogus comments in HTMLParser (GH-137873) (GH-137875)

Bogus comments that start with "<![CDATA[" should not include the starting "!"
in its value.
(cherry picked from commit 7636a66635a0da849cfccd06a52d0a21fb692271)

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