Gentoo packages for the old mailman 2.x
These may be used for now until mailman 3 is properly supported, but please note you're using these at your own risk and they're not in any way officially supported.
You may want to add this to your package.mask:
>=dev-python/dnspython-2.0.0
>=dev-python/pycryptodome-3.9.8-r1
>=dev-python/cffi-1.14.0-r3
>=dev-python/gmpy-2.0.8-r2
>=dev-python/ecdsa-0.15-r1
>=dev-python/six-1.15.0-r1
>=dev-python/idna-2.10-r1
>=net-mail/mailman-3.0