Prepare for 3.2.1 release.

We are going to move to more of SemVer now and avoid having to bump major versions (which, in our case, is minor version) if we don't need to. We will keep on bumping minor versions till we really need to bump major version due to backwards incompatibility reason.

This would also help us make more point releases that downstreams can upgrade to, without having to worry about bumping major versions. We don't really break API everytime when bumping major versions, which is slightly confusing and hard to keep track of.

This should also help us not worry too much about backporting fixes to old release branches. We will continue to release minor versions, till we have to bump major, after which the old version will be unmaintained. But in history of Mailman 3, we only had to break compatibility once with REST API change.

Finally, I'll keep the release codename of 3.2.0 series and keep 'Tom Swayer' reserved for the next major version bump, whenever we end up doing that.

This changes version to 3.2.1rc1