Fix code for python 3

Since dict.keys return a dictkeys under python 3, we hav to cast it
to a list to avoid traceback:

    Traceback (most recent call last):
      File "/tmp/ansible_sh16ejbd/ansible_module_authorized_key.py", line 496, in <module>
        main()
      File "/tmp/ansible_sh16ejbd/ansible_module_authorized_key.py", line 490, in main
        results = enforce_state(module, module.params)
      File "/tmp/ansible_sh16ejbd/ansible_module_authorized_key.py", line 410, in enforce_state
        parsed_new_key = parsekey(module, new_key)
      File "/tmp/ansible_sh16ejbd/ansible_module_authorized_key.py", line 308, in parsekey
        options = parseoptions(module, options)
      File "/tmp/ansible_sh16ejbd/ansible_module_authorized_key.py", line 259, in parseoptions
        options_dict[key] = value
      File "/tmp/ansible_sh16ejbd/ansible_module_authorized_key.py", line 164, in __setitem__
        self.itemlist.append(key)
    AttributeError: 'dict_keys' object has no attribute 'append'

Yet another fix for https://github.com/ansible/ansible/pull/18053

system/authorized_key.py

@@ -159,7 +159,7 @@ class keydict(dict):
 
     def __init__(self, *args, **kw):
         super(keydict,self).__init__(*args, **kw)
-        self.itemlist = super(keydict,self).keys()
+        self.itemlist = list(super(keydict,self).keys())
     def __setitem__(self, key, value):
         self.itemlist.append(key)
         super(keydict,self).__setitem__(key, value)