[3.13] gh-135661: Fix parsing attributes with whitespaces around the "=" separator in HTMLParser (GH-136908) (GH-136918)

This fixes a regression introduced in GH-135930.
(cherry picked from commit dee650189497735edbc08a54edabb5b06ef1bd09)

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