[Pkg-bazaar-maint] Bug#975463: breezy's autopkg tests fail with Python 3.9

[Matthias Klose]

Package: src:breezy
Version: 3.1.0-6
Severity: serious
Tags: sid bullseye ftbfs
User: debian-python at lists.debian.org
Usertags: python3.9

breezy's autopkg tests fail with Python 3.9:

[...]
AssertionError: not equal:
a = b'Requested to stop gracefully\n'
b = b"brz: ERROR: AttributeError: 'Thread' object has no attribute 'isAlive'\n"