Provide Python 3 requirenment hints
Provide Python 3 requirenment hints

This hopefully helps other admins trying to install modoboa to be careful about their Python version.
bearstech/modoboa,bearstech/modoboa,modoboa/modoboa,bearstech/modoboa,modoboa/modoboa,modoboa/modoboa,modoboa/modoboa,bearstech/modoboa