Feature/include findpython3 from cmake 3.19.3
I replaced the include FindPython3 code taken from master / will-be-3.20 to code from 3.19.3 and applied the adaptation to support 3.13 and then backportiong of "unversioned" Python from 3.20.
Fixes #233 (closed)
Edited by Christoph Grüninger