trim old Python2 compatibility layer

[a-detiste]

there's quite a lot left

[greyli]

Please fix the CI issues.

[a-detiste]

thanks, done

[a-detiste]

I think this shoud be solved in another PR.
(ie: unbind the precise slimit version)

  × Failed to build `slimit==0.8.1`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit
      status: 1)

      [stdout]
      Python 3.X support requires the 2to3 tool.

[greyli]

Merged, thanks!