Comment 2 for bug 1937173

Locally reproducible in sbuild of impish
Most obvious difference is a newer python 3.9.2-3 -> 3.9.4-1

A re-build in Debian unstable works fine

Even Re-runnning the test in Impish after a failed build (chrooted into the sbuild environment)

a) as a whole
BRZ_CONCURRENCY=3 BRZ_HOME=debian/brzhome BRZ_PLUGIN_PATH=-site:-user LC_ALL=C.UTF-8 PYTHONPATH=/build/breezy-xbGxjH/breezy-3.2.1/.pybuild/cpython3_3.9_breezy/build python3.9 ./brz -Derror selftest -v

b) and a formerly failing test individually
BRZ_CONCURRENCY=3 BRZ_HOME=debian/brzhome BRZ_PLUGIN_PATH=-site:-user LC_ALL=C.UTF-8 PYTHONPATH=/build/breezy-xbGxjH/breezy-3.2.1/.pybuild/cpython3_3.9_breezy/build python3.9 ./brz -Derror selftest -v breezy.tests.test_plugins.TestLoadingPlugins.test_plugin_with_error
/build/breezy-xbGxjH/breezy-3.2.1/breezy/tests/test_source.py:23: DeprecationWarning: The parser module is deprecated and will be removed in future versions of Python
  import parser
/build/breezy-xbGxjH/breezy-3.2.1/breezy/tests/test_source.py:25: DeprecationWarning: The symbol module is deprecated and will be removed in future versions of Python
  import symbol
running 2 tests...
brz selftest: /build/breezy-xbGxjH/breezy-3.2.1/brz
   /build/breezy-xbGxjH/breezy-3.2.1/breezy
   bzr-3.2.1 python-3.9.6 Linux-5.4.0-73-generic-x86_64-with-glibc2.33

test_plugins.TestLoadingPlugins.test_plugin_with_error OK 24ms
test_plugins.TestLoadingPlugins.test_plugin_with_error_suppress OK 4ms
----------------------------------------------------------------------
Ran 2 tests in 0.035s

OK

It only fails right at the build, and so far I have not found what makes the difference between this and the re-run.