Remove transitive dependency 'six' from setup.py

[vtalos]

As part of our ongoing research on Python dependency management we noticed a potential improvement in your project’s dependency management.

Specifically, the transitive dependency six is specified as an install requirement in the setup.py file, even though it is not used directly and does not need to be listed explicitly.

This PR removes it from setup.py to let pip manage it automatically, which helps keeping the dependency list clean.

Hope this is helpful!