Description of problem:
`dnf playground enable` fails with python traceback.

Version-Release number of selected component (if applicable):
# rpm -q dnf dnf-plugins-core 
dnf-1.1.1-2.fc23.noarch
dnf-plugins-core-0.1.11-1.fc23.noarch


How reproducible:
every-time

Steps to Reproduce:
1. #> dnf playground enable

Actual results:
You are about to enable a Playground repository.

Do you want to continue? [y/N]: y
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/python3.4/site-packages/dnf/cli/main.py", line 198, in user_main
    errcode = main(args)
  File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 84, in main
    return _main(base, args)
  File "/usr/lib/python3.4/site-packages/dnf/cli/main.py", line 136, in _main
    cli.run()
  File "/usr/lib/python3.4/site-packages/dnf/cli/cli.py", line 1085, in run
    return self.command.run(self.base.extcmds)
  File "/usr/lib/python3.4/site-packages/dnf-plugins/copr.py", line 382, in run
    self._cmd_enable(chroot)
  File "/usr/lib/python3.4/site-packages/dnf-plugins/copr.py", line 339, in _cmd_enable
    output = self._get_data(f)
  File "/usr/lib/python3.4/site-packages/dnf-plugins/copr.py", line 299, in _get_data
    output = json.loads(f.read())
  File "/usr/lib64/python3.4/json/__init__.py", line 312, in loads
    s.__class__.__name__))
TypeError: the JSON object must be str, not 'bytes'


Expected results:
playground enabled


Additional info:
Patch that works, but I'm not sure whether it shouldn't be fixed in dnfpluginscore/lib.py..

--- /usr/lib/python3.4/site-packages/dnf-plugins/copr.py.orig	2015-10-01 07:57:41.885483723 +0200
+++ /usr/lib/python3.4/site-packages/dnf-plugins/copr.py	2015-10-01 07:56:25.485292506 +0200
@@ -335,7 +335,7 @@ You are about to enable a Playground rep
 Do you want to continue? [y/N]: """)
         api_url = "{0}/api/playground/list/".format(
             self.copr_url)
-        f = dnfpluginscore.lib.urlopen(self, None, api_url)
+        f = dnfpluginscore.lib.urlopen(self, None, api_url, "w+")
         output = self._get_data(f)
         f.close()
         if output["output"] != "ok":