Porting to Python 3

by VPeric for SymPy

Python 3 represents the future of the language. If SymPy is to be used as a library, it will have to be ported to Python 3. In order to accomplish this, a system for automatic testing will need to be established. A robust test system will ensure no regression happen and will ease the maintaining of a dual code-base. After the main porting is done, my goal is to attempt to support alternative Python compilers, to further ensure compatibility and reduce reliance on implementation details.