[3.12] gh-135661: Fix parsing start and end tags in HTMLParser according to the HTML5 standard (GH-135930) (GH-136268)

* Whitespaces no longer accepted between `</` and the tag name.
  E.g. `</ script>` does not end the script section.

* Vertical tabulation (`\v`) and non-ASCII whitespaces no longer recognized
  as whitespaces. The only whitespaces are `\t\n\r\f `.

* Null character (U+0000) no longer ends the tag name.

* Attributes and slashes after the tag name in end tags are now ignored,
  instead of terminating after the first `>` in quoted attribute value.
  E.g. `</script/foo=">"/>`.

* Multiple slashes and whitespaces between the last attribute and closing `>`
  are now ignored in both start and end tags. E.g. `<a foo=bar/ //>`.

* Multiple `=` between attribute name and value are no longer collapsed.
  E.g. `<a foo==bar>` produces attribute "foo" with value "=bar".

* Whitespaces between the `=` separator and attribute name or value are no
  longer ignored. E.g. `<a foo =bar>` produces two attributes "foo" and
  "=bar", both with value None; `<a foo= bar>` produces two attributes:
  "foo" with value "" and "bar" with value None.

* Fix data loss after unclosed script or style tag (gh-86155).

Also backport test.support.subTests() (gh-135120).

---------
(cherry picked from commit 0243f97cbadec8d985e63b1daec5d1cbc850cae3)

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>
Co-authored-by: Waylan Limberg <waylan.limberg@icloud.com>