FindPython: use system Python 3 version if available
The newest installed version of Python 3 is often not the default version on a Gentoo Linux system, where multiple Python 3 versions can be installed on a single system and /usr/bin/python3 is the one that should be used by default.
The proposed patch (originally written by Bernd Buschinski) fixes the following bug on Gentoo Linux: https://bugs.gentoo.org/835799
... and it does not cause any problems on Linux distributions where /usr/bin/python3 is a symlink to the newest installed version of Python 3.