Use --force-flaky to decrease the need for tox re-runs

Author: nedbat

setup.cfg

@@ -1,5 +1,5 @@
 [tool:pytest]
-addopts = -q -n3 --strict --no-flaky-report -rfe --failed-first
+addopts = -q -n3 --strict --force-flaky --no-flaky-report -rfe --failed-first
 markers =
     expensive: too slow to run during "make smoke"
 # How come this warning is suppressed successfully here, but not in conftest.py??