Python2 is set to be deprecated (end of life) in 2020.
http://legacy.python.org/dev/peps/pep-0373/

5 years ahead is a reasonable time length to start porting bitbake and relevant code from Python2 to Python3.
Debian's project to port the Python codebase already started:
https://lists.debian.org/debian-devel-announce/2015/04/msg00005.html

Django 1.6 which is used for Toaster already runs on Python3:
https://docs.djangoproject.com/en/1.8/topics/python3/

Python's porting documentation:
https://docs.python.org/3/howto/pyporting.html

This bug tracks the work needed to run bitbake under Python3. 

I propose using the Python 2/3 Compatible Source strategy, as used by Django, that enables the same code base to work without modification on both Python2 and Python3.