Add the ability to specify the Python version in the CMake configuration.
Add the ability to specify the Python version in the CMake configuration.

thewtex/python-cmake-buildsystem,davidsansome/python-cmake-buildsystem,jcfr/python-cmake-buildsystem,python-cmake-buildsystem/python-cmake-buildsystem,adrianbroher/python-cmake-buildsystem,thewtex/python-cmake-buildsystem