BaseCommand, BaseMultiCommand, and BaseSubCommand now include

typing for all code, and modernized documentation.
This has required some small typing fixes throughout the codebase, but

has also found a few typing and logic errors that are being fixed. For

instance, BaseCommand._init_server_url() received a fix to avoid

crashing if repository information couldn't be found.
Some legacy Python 2 conditional code has been removed. There are some

other deprecated code paths that don't have proper typing, but those

will be going away in a separate change.

Unit tests pass.
Tested posting several review requests for review, including this one.