Introducing python3 into user system

[Ruslan Makhmatkhanov]

Hi,

Python 2.7 will have no upstream support after 2020, and right now it no 
more getting new features. So I'd want to start poisoning^W  introducing 
python3 into user system via ports tree and here is what I suggest to 
start to do right now:

- if port has a build dependency upon python and it can be built both 
with python2 and python3, force it to build with python3
- if some standalone application can be running both with python2 and 
python3 and it have equivalent python3 dependencies, force it to be run 
with python3

We also need to explicitly mark python2-only ports to simplify switching 
to python3 as default at some point of time.

What you guys think about? If there is general consensus, we may enforce 
this via Porters Handbook.

-- 
Regards,
Ruslan

T.O.S. Of Reality