Python 3.10 cleanups

[nbraud]

• README: Update for Py3.10+ support
• tests/helpers: Clean up Py3.6 cruft
• cli.main: update Python version check

[nbraud]

@duckinator, do you think we could reasonably make the version check in cli.main use the same source-of-truth as requires-python in pyproject.toml ?

I guess we could ship pyproject.toml in the wheel, and have main just parse it, but there's hopefully a more lightweight way to do it

[duckinator]

Opened #368 about investigating a single source of truth for minimum supported Python version. I'd like to do that, but think it might require some poking around to find a good solution.