BazaarClient has been updated to use the new run_process() instead

of execute(), providing better type safety and results, with more

thorough unit testing.
The functions in BazaarClient have also been updated to have type

annotations. This has already caught a few places where we made

assumptions about values that led to bad command invocations.
Some of this code now asserts the values that are being worked, to

ensure they're not None or the wrong type.
There's further work that may need to be done here.

parse_revision_spec() can fill in None values, which will now

trigger some later assertions, but we should probably raise exceptions

here. A TODO has been left in parse_revision_spec() to track this.

Unit tests pass on Python 3.7-3.11.