Remove elementtree deprecated methods

All our supported runtimes [1] are compatible with the recommended
alternatives.

`Element.getiterator` [2] is deprecated since python 3.2 and will be removed
in python 3.9, these changes switch usages to Element.iter() [3]

[1] https://governance.openstack.org/tc/reference/runtimes/victoria.html#python-runtimes-for-train
[2] https://docs.python.org/3/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.getiterator
[3] https://docs.python.org/3/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.iter

Change-Id: Iaf5291698d866c9c86cda0f255799d8f10fff3dd

os_ken/tests/integrated/test_of_config.py

@@ -235,7 +235,7 @@ class OFConfigClient(app_manager.OSKenApp):
         self._validate(tree)
 
         name_spaces = set()
-        for e in tree.getiterator():
+        for e in tree.iter():
             name_spaces.add(capable_switch.get_ns_tag(e.tag)[0])
         print(name_spaces)