craftenv.sh: use python3 (after version check) if python3.6 isn't found

Summary: On my system I no longer have python3.6 and python3 points to python3.7

Test Plan: craftenv.sh no longer complains about missing python3.6

Reviewers: vonreth

Reviewed By: vonreth

Differential Revision: https://phabricator.kde.org/D15075