specifically call python3, in scripts

with this change, it's unlikely we'll hit errors again. previously,
some projects used were calling "python" which in context was
python3, but on some setups, the user only has python2 and python3
but no symlink for "python" (which if exists, we assumed linked to
python3)

now it's unambiguous. docs/build/ can probably be updated now, as
a result of this change, to remove the advice about that