Make `FIND_PACKAGE(PythonInterp)` prefer `python3`

This change makes it possible to prefer a python (version 3) interpreter
on systems where python2 and python3 coexist (where python2 is found as
`/usr/bin/python`).

tests/CMakeLists.txt

+SET(Python_ADDITIONAL_VERSIONS 3 2.7)
 FIND_PACKAGE(PythonInterp)
 
 IF(NOT PYTHONINTERP_FOUND)