swig-python: add support for python 2.7/legacy

In principle this is a big hack as it modifies the python class at runtime. Since it's python legacy I hope it won't stay that long.

I was not able to let cmake compile bot python + python-legacy in one step. I fear that's not possible at all as both modules use the FindPythonLibs cmake module which will only run once.

So the best way to force compilation of python-legacy is to run cmake with some extra defines. I think you'll need to define:
* PYTHON_EXECUTABLE:FILEPATH=/usr/bin/python2.7
* PYTHON_INCLUDE_DIR:PATH=/usr/include/python2.7
* PYTHON_LIBRARY:FILEPATH=/usr/lib64/libpython2.7.so (or /usr/lib/libpython2.7.so)