Multiple python-3 packages

[Leo Famulari]

Currently we have Python 3.4 and 3.5. Python 3.6 was released recently.

When we added 3.5, I thought it was a good idea to keep 3.4 around in
case some packages need it. It turns out that nothing is using it in our
tree.

Do we want to keep 3.4 and 3.5 around? There is a maintenance cost
incurred at each new release, because their shared patches tend to
diverge each time.

If we should keep them, how should the package inheritance go? Now, it's
like this:

2.7 < 3.5 < 3.4

If we kept them all, should 3.5 and 3.4 both inherit 3.6?