Drop six from requirements

### Motivation
[six] is a package that helps shipping code that is compatible with Python 2 as well as with the newer Python 3. We are currently only supporting Python 2.7 and it will be a long road before we support Python 3. Currently `six` is only used in just one line of code, which can be easily replaced with Python2 code. Since we aren't supporting Python 3, I propose to drop from our explicit requirement. Any other packages that depend on `six` (such as `pylint`) may download and use their own version.

### Changelog
* Replace all uses of `six` with plain Python (it's only one line, actually).
* Drop `six` from requirements.