cmake: use default Python version

If the Python versions are specified, this breaks the build on Ubuntu systems
where Python3 is also installed but the extensions such as python-empy
are not installed. One could, of course, install python3-empy to fix
this but that's not in the instructions or error messages and therefore
not straightforward.

It is therefore probably better to just use the system default which
ends up being 2.7 on Ubuntu.