Description of problem:
I tried to update fedora 20-0.8 with dnf, but it cannot be performed.
It works with yum.

Version-Release number of selected component (if applicable):
sh-4.2# rpm -q fedora-release
fedora-release-20-0.8.noarch
sh-4.2# rpm -q dnf
dnf-0.4.8-1.fc20.noarch

How reproducible:
allways

Steps to Reproduce:
1. Make sure, you have older version of fedora 20 (e.g. 20-0.8)
2. Try to upgrade fedora with dnf
   dnf update -y fedora-release

Actual results:

sh-4.2# dnf update -y fedora-release
Resolving dependencies
--> Starting dependency resolution
---> Package fedora-release.noarch 20-0.8 will be upgraded
---> Package fedora-release.noarch 20-1 will be an upgrade
--> Finished dependency resolution
Dependencies resolved.

==========================================================================================================================================================================================
 Package                                            Arch                                       Version                                   Repository                                  Size
==========================================================================================================================================================================================
Upgrading:
 fedora-release                                     noarch                                     20-1                                      fedora                                      33 k

Transaction Summary
==========================================================================================================================================================================================
Upgrade  1 Package

Total size: 33 k
Downloading Packages:
[SKIPPED] fedora-release-20-1.noarch.rpm: Already downloaded                                                                                                                             
warning: /var/cache/dnf/x86_64/20/fedora/packages/fedora-release-20-1.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 246110c1: NOKEY
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-20-x86_64
Traceback (most recent call last):
  File "/usr/bin/dnf", line 35, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 279, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 192, in _main
    return_code, resultmsgs = base.do_transaction()
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 198, in do_transaction
    if self.gpgsigcheck(downloadpkgs) != 0:
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 233, in gpgsigcheck
    self.getKeyForPackage(po, fn)
  File "/usr/lib/python2.7/site-packages/dnf/base.py", line 2263, in getKeyForPackage
    keys = self._retrievePublicKey(keyurl, repo)
  File "/usr/lib/python2.7/site-packages/dnf/base.py", line 2159, in _retrievePublicKey
    rawkey = dnf.util.urlopen(keyurl, repo).read()
ValueError: I/O operation on closed file



Expected results:
Pacakge will be upgraded

Additional info:
It works with yum


sh-4.2# yum update -y fedora-release
Resolving Dependencies
--> Running transaction check
---> Package fedora-release.noarch 0:20-0.8 will be updated
---> Package fedora-release.noarch 0:20-1 will be an update
--> Finished Dependency Resolution

Dependencies Resolved

==========================================================================================================================================================================================
 Package                                            Arch                                       Version                                   Repository                                  Size
==========================================================================================================================================================================================
Updating:
 fedora-release                                     noarch                                     20-1                                      fedora                                      33 k

Transaction Summary
==========================================================================================================================================================================================
Upgrade  1 Package

Total download size: 33 k
Downloading packages:
warning: /var/cache/yum/x86_64/20/fedora/packages/fedora-release-20-1.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 246110c1: NOKEY
Public key for fedora-release-20-1.noarch.rpm is not installed
fedora-release-20-1.noarch.rpm                                                                                                                                     |  33 kB  00:00:00     
Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-20-x86_64
Importing GPG key 0x246110C1:
 Userid     : "Fedora (20) <fedora>"
 Fingerprint: c7c9 a9c8 9153 f201 83ce 7cba 2eb1 61fa 2461 10c1
 Package    : fedora-release-20-0.8.noarch (@Fedora-20-x86_64-main)
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-20-x86_64
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Updating   : fedora-release-20-1.noarch                             1/2 
  Cleanup    : fedora-release-20-0.8.noarch                           2/2