Drop use of six

[s-t-e-v-e-n-k]

This package has an undeclared requirement on six, but since Python 2 is long unsupported, remove it and just use the Python 3 idiom.

[a-detiste]

It's broken on Debian too, I'm using your patch
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1084379

[a-detiste]

More specificaly: I broke it indirrectly by removing six elsewhere

[s-t-e-v-e-n-k]

      File "/<<PKGBUILDDIR>>/sklearn_pandas/pipeline.py", line 1, in <module>
        import six
    ModuleNotFoundError: No module named 'six'

This appears that you haven't applied the patch, that's the only file this PR changes.

[a-detiste]

I mean, it's moslty likely visible because I patched-out six from dateutil or some other low level library that was pulled in with sklean-pandas
https://wiki.debian.org/Python3-six-removal

sklearn-pandas is patched & uploadded right now
https://salsa.debian.org/science-team/sklearn-pandas/-/pipelines/814959