Raise supported version to Python 3.11

[crapitea]

At the present time, there is no reason why the maximum supported version shouldn't be 3.11.

[crapitea]

Tox execution output:

  py36: SKIP (0.75 seconds)
  py37: SKIP (0.00 seconds)
  py38: SKIP (0.00 seconds)
  py39: SKIP (0.00 seconds)
  py310: OK (52.62=setup[17.15]+cmd[30.83,0.00,0.26,0.44,3.13,0.80] seconds)
  py311: OK (28.52=setup[10.74]+cmd[13.76,0.00,0.25,0.36,2.66,0.74] seconds)
  congratulations :) (81.95 seconds)

[nassauwinter]

Approved and ready to merge.