Description of problem:
When trying to upgrade from Fedora 27 to 28 dnf system-upgrade fails because of an unresolved dependency on libevent 2.0. The Fedora 28 repos seem to only provide libevent 2.1 at the moment which does not match opensmtpd-6.0.3p1-2.fc28.x86_64's Requires stanza (libevent-2.0.so.5()(64bit)).
This situation probably also prevents the package from being installed on a F28 system in the first place (untested).

The opensmtpd package should probably be rebuilt against libevent-2.1 or an additional package providing libevent-2.0.so.5()(64bit) should be in the repositories.

Version-Release number of selected component (if applicable):
opensmtpd-6.0.3p1-2.fc28.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Have a up-to-date Fedora 27 with opensmtpd-6.0.3p1-1.fc27.x86_64 installed
2. Try to do a system-upgrade via `dnf system-upgrade download --releasever=28`

Actual results:
dnf fails to upgrade opensmtpd and thus the release version with:

```
 Problem: problem with installed package opensmtpd-6.0.3p1-1.fc27.x86_64
  - package opensmtpd-6.0.3p1-2.fc28.x86_64 requires libevent-2.0.so.5()(64bit), but none of the providers can be installed
  - opensmtpd-6.0.3p1-1.fc27.x86_64 does not belong to a distupgrade repository
  - libevent-2.0.22-6.fc27.x86_64 does not belong to a distupgrade repository
```

Expected results:
opensmtpd and therefor the system's release version are correctly upgraded.

Additional info:
Judging by the info on Koji (https://koji.fedoraproject.org/koji/packageinfo?packageID=761) the version of libevent was bumped after the F28 mass rebuild (rebuilt on 2018-02-09; bumped to 2.1 on 2018-02-14). So this issue could potentially impact more dependent packages.