add Python type hints to piuparts.py and its dependencies

In order to fix a few type errors, some semantic changes are included.

• Methods of the Defaults class raise NotImplementedError rather than returning None.
• The settings class assigns default values for apt_unauthenticated, distro_config and testobjects in its constructor.
• Renamed a few variables that were used with different types.
• Added a number of assert something is not None to guide mypy.
• get_state_meta_data constructs its result at once.
• TimeOffsetFormatter.formatTime signature updated to match super class: Added default value.
• The type of the warn_only argument changed to bool in two methods.
• Added missing return statements and None values to existing return statements.
• find_default_debian_mirrors returns [] when it previously returned None.
• Rewrote DistroConfig.get as get_field to avoid conflicting with the super class type.
• DistroConfig.get_deb_lines caches a lookup to help mypy track state.
• unqualify always returns a list now.