Update some Python scripts to reference `python3` instead of `python`.

Update some Python scripts to reference `python3` instead of `python`.

Python 2 is no longer supported upstream, so we really should default to
running Python 3 for all code here.  Update most scripts to reference that.

Most code should have already been converted to be compatible with Python 3
and no further changes were made here, except for one fix needed in
`tests/digest_tree`.  Note: Changes have not yet been much tested, so there
could possibly be bugs.

A few scripts in `contrib/` are not updated as these would need some extra
work; leave that for the future.

Python 2 compatibility code could probably be removed (the `six` module and
`__future__` imports), but also leave that for future work to keep this
commit small.