Easy switch from python2 to python3

[mdavezac]

PEP states that in the future the python and python2 command should always point to python 2 and the python3 command should always point to python 3.
Would it be possible to have such a thing in the CMakeCookOff and via this get a switch to configure either for Python 2 or for Python 3.

[mdavezac]

@tbetcke, that sounds like a good idea.
Currently, I'm just instantiating virtualenvs with either python2 or python3 to test one or the other. But that's more cumbersome than it has to be.

[mdavezac]

So, it looks like the following does the trick:

cmake -DPythonInterp_FIND_VERSION=2 -DPythonInterp_FIND_VERSION_MAJOR=2 ..

But we could make it easier, either from the cookoff or from bempp.