Incompatibility with Python 3.4 & 3.5
As it stands, the master build of Mailman 3 is entirely incompatible with Python 3.4 & 3.5. The fix is simple though. In setup.py, aiosmtpd>=1.0a5 currently resolves to aiosmtp 1.1, which has a strict dependency on features new to python 3.6. Changing this dependency to aiosmtpd==1.0 is probably warranted.