Modify CMake to allow building for a specific (or many at once) version of Python

[vmoeykens]

Right now, the CMakeLists.txt file is set up to simply find a valid python interpreter on your system, and build for it. However, we would like at least the functionality to specify a specific version of python to build for, and ideally we would like to build for many at once. Due to pybind11 checks, we can't simply use any Python 3 binary, we have to use specific ones.

This Github issue may provide some insight into how to build for multiple versions of Python at once: pybind/pybind11#748