Add note for compiling eigenpy with python3 on macOS

Getting eigenpy (and pinocchio) to compile on macOS with python3 took me some effort. The required trick turned out to "only" add `-DPYTHON_EXECUTABLE=\`which python3\`` to the cmake argument.

Do you think this is worth adding to the README.md/installation instructions? I am happy to provide something similar for pinocchio as well.