chore: Remove usage of six package

[mattkram]

The six library has been imported for a long time, as a result of the transition from python 2 -> 3. But we are way past that point, and anaconda-client already declares support for python<=3.8.

As a result, there is some dead code we can remove, all dependent on imports from six. This PR removes all references to six, and also removes some dead code which isn't referenced from anywhere inside the binstar_client package.

[AlbertDeFusco]

Some pylint errors?

[mattkram]

I will need to re-think some of the removals here. Apparently anaconda-project has a dependency on this requests_ext module: https://github.com/anaconda/anaconda-project/blob/6968dda6e286e0a714fa85ec812df4e86e2d5136/anaconda_project/client.py#L129