Replace all references to Distribute with Setuptools.

[artempyanykh]

There is some inconsistency in the docs related to Distribute and Setuptools. In one place you can see Distribute, in the other place - Setuptools.

Distribute was merged back into setuptools and discontinued.

Related issues are: #329, #289.

[sigmavirus24]

I'm not convinced that this change or the similar one in the diff below is accurate. These seem to be explicitly talking about pre-requisites for using the tools bbFreeze and PyInstaller. I will have to check if these actually can use setuptools or whether the references have to be to distribute. For what it is worth, there was effort made to make setuptools and the last version of distribute interchangable but for many users (and projectes) they are not. There are also several issues with one or two of the most recent versions of setuptools and prior versions.

[artempyanykh]

Before doing PR, I took a quick look on the topic, and it seemed that bbFreeze and pyInstaller can be installed with setuptools.

But if I was mistaken, I can fix the PR.

[kennethreitz]

✨ 🍰 ✨

[kennethreitz]

long overdue btw :)