bpo-30365: Backport warnings and fix bugs in ElementTree. (#1581)

bpo-30365: Backport warnings and fix bugs in ElementTree. (#1581)

Running Python with the -3 option now emits deprecation warnings for
getchildren() and getiterator() methods of the Element class in the
xml.etree.cElementTree module and when pass the html argument to
xml.etree.ElementTree.XMLParser().

Fixed a deprecation warning about the doctype() method of the
xml.etree.ElementTree.XMLParser class.  Now it is emitted only when
define the doctype() method in the subclass of XMLParser.

Fixed a bug in the test_bug_200708_close test method.  An EchoTarget
instance was incorrectly passed to XMLParser() as the html argument and
silently ignored.

Tests no longer failed when use the -m option for running only selected
test methods. Checking warnings now is more specific, warnings are
expected only when use deprecated features.