* Ensure unclosed script tags are parsed correctly by providing a workaround for https://bugs.python.org/issue41989.
* Avoid cdata_mode outside of HTML blocks, such as in inline code spans.

Fixes #1036.

* All non-language classes should always be assigned to the pre tag.
* The language identifying class should never be included with the
general list of classes.

Fixes #1032

Python 3.5 reached end-of-life on 2020-09-12 and Python 3.9 was released on 2020-10-05.

A second function, `slugify_unicode` was added rather than changing the existing function so as to maintain backward compatibility. While an `encoding` parameter was added to the `slugify` function, we can't expect existing third party functions to accept a third parameter. Therefore, the two parameter API was preserved with this change.

The HTML parser has been completely replaced. The new HTML parser is built on Python's html.parser.HTMLParser, which alleviates various bugs and simplifies maintenance of the code.

The md_in_html extension has been rebuilt on the new HTML Parser, which drastically simplifies it. Note that raw HTML elements with a markdown attribute defined are now converted to ElementTree Elements and are rendered by the serializer. Various bugs have been fixed.

Link reference parsing, abbreviation reference parsing and footnote reference parsing has all been moved from preprocessors to blockprocessors, which allows them to be nested within other block level elements. Specifically, this change was necessary to maintain the current behavior in the rebuilt md_in_html extension. A few random edge-case bugs (see the included tests) were resolved in the process.

Closes #595, closes #780, closes #830 and closes #1012.

Closes #1019.

As of richleland/pygments-css@146834e1 the css class was set to `.highlight`.
Closes #1020.