clear some Python 2 remnants (including six)

[MeggyCal]

Thanks for all your work on this library! As it is already Python 3 only, it shouldn't need the six translation layer, which will die with Python 3.13.

[cwacek]

@MeggyCal can you run black on this and push the update.

$ pip install black==23.7.0
$ black --extend-exclude=versioneer.py .

[MeggyCal]

Hi, done (with black 24.3.0, I hope the result will be equivalent).