Update information about Python 3

Python 3 and Flask is a good combination nowadays that works rather well. In fact, [most PyPI libs](https://regebro.wordpress.com/2014/10/23/59-of-maintained-packages-support-python-3/) are now ported and it's generally ok to encourage people to use Python 3 for new projects.
Admit the occasional encoding problem
Describe the nature of Python 3 roughness
Remove head lines and add versions of first Python 3 support
Merge pull request #1264 from svenstaro/patch-1

Update information about Python 3