Remove Python 2.6 and <3.4, Add Python 3.6 and flake8
Remove Python 2.6 and <3.4, Add Python 3.6 and flake8

* Remove Python 2.6, 3.2, and 3.3 because they are now [end of life](https://devguide.python.org/#branchstatus).
* Add Python 3.6
* Test with [__flake8__](http://flake8.pycqa.org/) and stop the build if there are Python syntax errors or undefined names
drj11/pypng,drj11/pypng