[3.11] gh-135661: Fix parsing attributes with whitespaces around the "=" separator in HTMLParser (GH-136908) (GH-136920)

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

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

• Lib/html/parser.py[diff]
• Lib/test/test_htmlparser.py[diff]
• Misc/NEWS.d/next/Security/2025-06-25-14-13-39.gh-issue-135661.idjQ0B.rst[diff]