Bazaar is long-abandoned, with no formal Python 3 support. However, it

does have a modern fork called Breezy, which we've previously added some

initial support for in RBTools.
Breezy is largely compatible with Bazaar, but does use its own Python

modules, command line tool name, and environment variables. Review Board

mostly worked with Breezy, so long as bzr was symlinked to brz in

the system path, but our rbssh plugin could not be invoked due to

changes in environment variables.
This half-measure of support caused unit tests in an environment without

bzrlib to fail, as the tests would see bzr and would run, but the

rbssh plugin would fail at import time. This effectively skipped the

plugin, causing Breezy to use default SSH support, which would fail one

of the unit tests dealing with SSH key management.
This change introduces official Breezy support. We now check for the

presence of brz. If found, we assume Breezy support, and adjust

settings accordingly. Otherwie, we assume brz support, for legacy

purposes.
There isn't really a whole lot that has changed, other than using the

proper name and environment variables. However, going forward, there is

an important caveat with the plugin. This needs to remain compatible

with Python 2 and 3, since it's run in either the Bazaar or Breezy

process. It also needs to import the correct module (bzrlib or

breezy).
breezy is now a development dependency when running on Python 3.
This change is being introduced for Review Board 5, but is planned to

be backported to 4.0.7.

Unit tests pass with Bazaar and Breezy.