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

[miss-islington]

• 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 (htmlparser unclosed script tag causes data loss #86155).

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

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(cherry picked from commit 0243f97)
(cherry picked from commit c555f88)

Co-authored-by: Serhiy Storchaka storchaka@gmail.com
Co-authored-by: Ezio Melotti ezio.melotti@gmail.com
Co-authored-by: Waylan Limberg waylan.limberg@icloud.com

• Issue: HTMLParser differences from the HTML5 specification #135661