The proposed PR removes old-deprecated ElementTree features.

* Methods Element.getchildren(), Element.getiterator() and ElementTree.getiterator() deprecated in 2.7 and 3.2. They were deprecated in the documentation only, and started to emit a warning in 3.8. Use list(elem) or iteration instead of getchildren(), methods iter() instead of getiterator().

* The xml.etree.cElementTree module deprecated in 3.3. It was deprecated documentation only because adding a runtime warning will cause more harm than removing it because of the common idiom of using it since Python 2:

    try:
        import xml.etree.cElementTree as ET
    except ImportError:
        import xml.etree.ElementTree as ET

TODO: Add a What's New entry after the start of developing 3.9.