Python Version Support, lxml[html_clean] fix, docs fix

[dbowring]

• Fixes support for lxml>=5.2.0
• Adds explicit support for current python versions
• Removes support for EOL'd python versions
• Removes support for EOL'd platforms
• Fixes build for documentation
• Replaces use of recommonmark with myst-parser for markdown in docs

Replaces #186 (I promise this is the last revision 😅 ). This one offers each change as an individual commit. This does not migrate to pyproject.toml, the addition of lxml-html-clean; python_version < '3.11' is sufficient to get the tests and installation running again.

[sanand0]

@buriy -- this PR from @dbowring would be of help to those running readability on newer Python versions (3.11 in my case). Could you please take a look? Thanks!