Replace mercurial-server by hg-ssh
mercurial-server does not seem to be maintained anymore and it has no support for Python3 (it will be masked on Gentoo in 30 days). I think it will be better to use hg-ssh. For that we need:
* update tryton-tools/roundup_sshkeys.py to generate a proper .ssh/authorized_keys for user hg.
* patch (and propose upstream) to set HGUSER environment per key like I did for mercurial-server [1] to support acl.
[1] https://bitbucket.org/lshift/mercurial-server/pull-requests/5/set-user-as-login-name/diff