[3.13] gh-69426: HTMLParser: only unescape properly terminated character entities...

[3.13] gh-69426: HTMLParser: only unescape properly terminated character entities in attribute values (GH-95215) (GH-133586)

According to the HTML5 spec, named character references in attribute values
should only be processed if they are not followed by an ASCII alphanumeric,
or an equals sign.
(cherry picked from commit 77b14a6d58e527f915966446eb0866652a46feb5)

https: //html.spec.whatwg.org/multipage/parsing.html#named-character-reference-state

Co-authored-by: Sascha Ißbrücker <sascha.issbruecker@googlemail.com>