mail/mailman3: update to 3.3.8-81-g5e4431af6
This update mail/mailman3 to intermediate commit 3.3.8-81-g5e4431af6 to
avoid a dependency on py-importlib-resources. Needed since
py-importlib-resources has been upgraded to 6.0.1 which is incompatible
with mailman.
PR: 273259