Proposed patch removes old-deprecated ElementTree features.

* Methods Element.getchildren(), Element.getiterator() and ElementTree.getiterator() deprecated in 2.7 and 3.2.
 Use list(elem) or iteration instead of getchildren(), methods iter() instead of getiterator(). 

* The html argument of XMLParser deprecated in 3.4.
 The rest of arguments are keyword-only now (passing them as keywords was recommended in the documentatin).

* The support of the doctype() method of XMLParser subclasses and its default implementation. Deprecated in 3.2. Define the doctype() method on a custom TreeBuilder target instead.

* The xml.etree.cElementTree module deprecated in 3.3.

Unfortunately some of these deprecations are in the documentation only or in Python implementatation, but not in C implementatation. Perhaps missed warnings should be added first (see issue29204). But if commit the patch from issue29204 in 3.6, perhaps deprecated features could be removed in 3.7.