get-poetry.py fallback to standard executables (#1878)

* get-poetry.py fallback to standard executables

* documentation update

* fix typo due to cleanup

* favor python3 over python2 when creating launcher

* launcher favors 3.5 and up, else the first valid one it finds.

* favor any py3 over py2, not just 3.5+

Co-authored-by: Jonathan Piché <jpiche@coveo.com>