Drop Python 3.6 support

[pauloxnet]

What's the problem this feature will solve?

Python 3.6 is EOL since December 2021

Describe the solution you'd like

Drop Python 3.6 support in pip-tools

Additional context

There's already a related PR #1560

[gopackgo90]

There would need to be more changes than just what's in that PR, like updating python_requires in setup.cfg.

[pauloxnet]

Yes, I know, in fact I wrote that there is a PR related to the issue and not a final PR.

[hugovk]

Please see PR #1564 for the other changes.

[pauloxnet]

I commented there, but there are still some test failures there.

[hugovk]

Thanks, it's been included in the consolidated #1567 where tests now pass.