This switches swig to use python 3 specific features when generating the
bindings, switches the build over to python 3 and makes the neccesary
python 2 to 3 conversions in the bindings and tests.
Where possible in the Python SWIG code use the builtin SWIG conversion
code over custom code. This ensures appropriate overflow/type checking.
With this I have enabled GncNumeric from longs and tested for correct
overflow handling.
Note: This could be extended to GUILE but I am not familiar enought to
safely enable this.
