Because of flufl.i18n, the release-3.1 branch depends on Python 3.5+

The release-3.1 branch uses the _.defer_translation() method that appeared in flufl.i18n 2.0. However, the NEWS.rst file for this version also states that flufl.i18n has dropped support for Python 3.4 and now requires 3.5+.

Do we want to keep the Mailman 3.1 branch compatible with 3.4? It would make my life as a packager much easier, but I don't know how much work it represents.