[docs] [issue16331] Add a version switcher to python docs site

[Yury Selivanov]

Yury Selivanov added the comment:

> I mostly want to switch between 2.x and 3.x, and several modules got renamed between these versions.  There are also new modules on each release (5 new ones in 3.3).

To make that work we'll need to have a modules mapping between 2nd and 3rd versions.  It will also make URL parsing/changing logic more complicated and error prone...

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