bitbake: toaster: use decode('utf-8') for binary data

Decoded binary data to utf-8 to fix the following
error when runnign on python 3:
TypeError: Type str doesn't support the buffer API

[YOCTO #9584]

(Bitbake rev: 752ea00919ef054622a51ce097923309934eff2b)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/toaster/orm/models.py

@@ -1169,7 +1169,7 @@ class LayerIndexLayerSource(LayerSource):
             except URLError as e:
                 raise Exception("Failed to read %s: %s" % (path, e.reason))
 
-            return json.loads(res.read())
+            return json.loads(res.read().decode('utf-8'))
 
         # verify we can get the basic api
         try: