- Move the support modules inside python3-flightgear/flightgear/ and
remove their shebang, if any.
- Accordingly adapt the import statements.
- Change shebangs from e.g. '#! /usr/bin/python' to
'#! /usr/bin/env python3'.
- Small changes to make Python 3 happy with all scripts.
catalog/check_aircraft.py should be run under Python 3 from now on.
