Review requirements / docs

ujson being upgraded to 5.2.0, implicitly deprecates Python 3.6 (default for Ubuntu 18.04 LTS) in favor of Python 3.8 (default for Ubuntu 20.04 LTS)
Which is OK, I think, because we are deprecating Ubuntu 18.04 LTS support
(It's not viable to spend our efforts to support multiple distribution / versions,
given that we live in the era of dockerization and virtual machines)
If anything, we will move towards supporting upcoming Ubuntu 22.04 LTS.