Using the principles of continuous delivery, port to python 3 should progress as following:

• the first PR will add allow travis to run python3 while successfully doing nothing. The pybitmessage binary will check if it's running python 3 and if yes, exit with return code 0. Unit tests will be decorated to skip all tests when using python 3. This should allow travis to succeed in python 3. Not sure what to do if requirements.txt diverge between python 2 and python 3.
• gradually tests will be updated to allow running in python 3
• maybe new tests will be written to cater for areas that currently don't have tests to facilitate gradual porting
• once everything is working correctly in python 3, as well as binary builds for the main platforms, python 2 support will be dropped