make python 3 compatible again

I have done some fixes which gives python 3 compatibility back to `sure`!

The `function` objects in python 3 have no `func_code` member anymore. Use `__code__` instead.
The `byte` objects in python 3 have no `format` method. Use `encode` instead.
The representation of the strings and encoded strings are different in python 2 and python 3 thus use PY3 from six.
Remove unluckily example from README since python 3 returns a float and python 2 an int in this situation

**Python 2 and 3 build is working again!**

:beers: