bcrypt 3.2 dropped support for Python 2.7. This wasn't an issue if

installing Review Board via pip, but if using easy_install, the

wrong version would be picked up.
The 3.1.x series still supports Python 2.7, so we're now pinning that.
We don't directly depend on bcrypt, but cryptography and paramiko

do. When we drop Python 2.7 support, we can remove this dependency

completely.

Tested an installation on Python 2.7, and verified that bcrypt 3.1.7

was installed.