Fix parsing start and end tags in :class:`html.parser.HTMLParser`
according to the HTML5 standard.

* 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`` and space.

* 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".
