ADMIN: Upgrade to mailman 3

The current mailman software is not easy to navigate, viewing the [pandas-dev archives](https://mail.python.org/pipermail/pandas-dev/) is quite tough, whereas using mailman 3 is much more pleasant, it is very easy to navigate through the [numpy-discussion archives](https://mail.python.org/archives/list/numpy-discussion@python.org/) which is on mailman 3.