Fixes https://github.com/certbot/certbot/issues/8134.
* Test on Python 3.9.
* Mention Python 3.9 support in changelog.
* s/\( *'Pro.*3\.\)8\(',\)/\18\2\n\19\2/
* undo changes to tox.ini
* Move more tests to Python 3.9
* Update PyYAML and packages which pinned it back
* Upgrade typed-ast
* Use <= to "pin" dnspython
* Fix lint by telling pylint it cannot be trusted
* Disable mypy on RFC plugin
* add comment about <= support