Remove support for Python < 3.8

[Kami]

• Update .circle config and don't run tests on unsupported configurations anymore
• Remove obsolete targets and configurations from tox config
• Remove Python 2 and Python 3 < 3.8 compatibility code
• Remove modernize checks
• Remove any vendored 3rd party modules we don't need anymore
• Remove conditional python_version requirements in requirements.txt files
• Use Python 3.8 (3.9?) for all the Docker images
• Remove ujson dependency and use orjson everywhere