Remove six as a dependency

[datapythonista]

xref #909 #1040

Once #1040 is finished, the only main thing missing to completely drop support for python 2 is remove six from the list of dependencies. This is in setup.py, requirements-dev.txt, environment.yml (if it exists), the docs and anywhere else. A grep -R "six" * should help find any other appareance.

Once this is merged, we should probably close #909 too.

CC: @dorothykiz1

[dorothykiz1]

Yes this is noted @datapythonista

[datapythonista]

Fixed in #1136