test: avoid a 3.11.0b2 bug

Author: nedbat

setup.cfg

@@ -2,7 +2,9 @@
 # For details: https://github.com/nedbat/coveragepy/blob/master/NOTICE.txt
 
 [tool:pytest]
-addopts = -q -n auto --strict-markers --no-flaky-report -rfEX --failed-first
+# Use --assert=plain since 3.11.0b2 has a bug: https://github.com/pytest-dev/pytest/issues/10008
+# fixed by https://github.com/python/cpython/pull/93359
+addopts = -q -n auto --strict-markers --no-flaky-report -rfEX --failed-first --assert=plain
 python_classes = *Test
 markers =
     expensive: too slow to run during "make smoke"