Add Python 3 support using PJSUA2 API

[riza]

Currently we've been able to generate SWIG binding to java and python 2 using PJSUA2 API.

This ticket will add support to generating python 3 binding and enable it
as the preferred python binding.

To change it back to python 2 binding, you need to modify pjsip-apps/src/swig/python/Makefile to:

USE_PYTHON3=0

and regenerate the SWIG binding afterwards.

Note that when generating python 3 binding, you would require the similar packages used when generating python 2 binding.

e.g: python3-dev instead of python2.7-dev package

[riza]

In 5638:

Close #2034: Add support to Python3 using PJSUA2 API.