v6.0.0a0

This major release marks the migration to Python 3.7, removing Python 2 support. This release also pulls in the latest of dependencies.

Dependency version constraints have intentionally been removed from setup.py to give projects pulling in nrfutil leeway and responsibility of ensuring compatibility and freezing dependencies. Unfortunately this means that installing nrfutil standalone from the egg may break anytime and require manually fixing dependencies. A new file requirements-frozen.txt has been added which freezes the dependency environment from time of testing and should always work.