bin/python3 symlink for python3X ports

[Dmitry Sivachenko]

Hello!

It is common to have both python-2 and python-3 packages installed.
To help transition it is convenient to have bin/python to point to python-2 binary.

Now for every X in python3X we have a bin/python3.X binary.

So for every new python3.X release one should change shebang line in scripts.

I propose to tweak python3X ports so they create bin/python3 symlink pointing 
to the installed python3.X binary so that people can always use 'python3' name.

What do you think?

Thanks!