Switch to aiosmtpd
Core uses lazr.smtptest for various things, which is based on the stdlib smtpd module, which itself is based on asyncore/asynchat. It also uses an LMTP server based on smtpd. Well, all that technology is old and cruft, and we have shiny new goodness in aiosmtpd. We need to switch to aiosmtpd on the road to fixing Python 3.6 support in #295 (closed)