aiosmtpd seems abandoned and blocks Python 3.10 support
Unfortunately, it seems like aiosmtpd has gone dormant, which is a bit of a problem, since the test suite fails on Python 3.10.
This has basically blocked building Mailman for Fedora Linux 35, since we're shipping Python 3.10 as the default there.
I haven't been able to figure out why the tests fail to fix it myself, but if it's not fixed and nobody is doing anything, it may make sense to remove aiosmtpd as a dependency...