net-mail/mu-0.9.18 has been released. It looks like the only necessary change in the ebuild is that SRC_URI is no longer prefixed with "v" -- it's just "https://github.com/djcb/mu/archive/0.9.18.tar.gz".
commit 6915ecf5aed22ff1875774369128cd470fc91169 Author: Vikraman Choudhury <git@vikraman.org> Date: 33 seconds ago net-mail/mu: Bump to 0.9.18 and eapi 6 Package-Manager: Portage-2.3.3, Repoman-2.3.1