OK. I've attached a patch which removes the use of get_unused_port for test_smtplib and test_multiprocessing. It doesn't use bind_port as both cases test classes that create their own port internally, rather I've used port 0 and then introspected the actual port assigned.

The same process could probably be used for the remaining uses of get_unused_port, but I haven't tackled them yet.

Tested on my local PC and on the buildbot where I see the bug. Both tests run fine in both cases. I'm running the full test suite now.