1. Python 2.7 is a hard requirement. We’ve been working on the Python 3 upgrade for a couple years now, and are aiming to support it fully in Review Board 4.0 (a big part of the delay in this release). See
this writeup on our blog for more info.
3. Review Board itself does not require an entry in the hosts file, but this sounds like you're discussing domains and DNS registration more generally? If so, this is a bit outside the scope of the community support here. If you're installing a server within a company intranet, you are going to need to figure out who to talk to there about any internal DNS mappings. If you're installing just locally for testing, with the idea of eventually having a domain pointing pointing to that server, you can use an entry in the hosts file for now, but don't forget you put it there. Nobody else outside your machine will be able to use that domain.
Christian