Note that "libevent" seems to be the correct package for Amazon Linux:
libevent.i686 2.0.21-4.19.amzn1 amzn-main
Amazon Linux is odd in that its initially RHEL 6 based, but nowadays has slightly different packages.
Previously the pgbouncer package worked fine, so this seems like an unfortunate regression due to the explicit requirement in the package definition. Maybe this can be special cased in some way?