Convert python2 scripts to python3

We have various scripts written in python that should be converted to expecting python2 to expecting python3.
This includes the CLDR digestion scripts in util/locale_database/; we should check for others, too.
We need to decide which version of Python 3 to upgrade to.

This would also be a good moment for a flake8 run and a general clean-up of the coding style; when rewriting the util/locale_database/ scripts last year, I perpetrated some admittedly ediosyncratic stylistic deviations from PEP8, along with some deliberate choices to apply Qt coding style carried over from C++; some of the latter (e.g. naming convention following camelCase instead of snake_shaped) may be worth preserving, but the ediosyncracies probably aren't !