Fix "dnf list upgrades" parsing

The header for the output of this command appears to have changed at
some point, which breaks our output parsing. This adds the new header to
the `_strip_headers()` helper so that we ignore it while parsing.

salt/modules/yumpkg.py

@@ -85,6 +85,7 @@ def _strip_headers(output, *args):
     if not args:
         args_lc = ('installed packages',
                    'available packages',
+                   'available upgrades',
                    'updated packages',
                    'upgraded packages')
     else: