bpo-29204: Emit warnings for already deprecated ElementTree features. (#773)

Element.getiterator() and the html parameter of XMLParser() were
deprecated only in the documentation (since Python 3.2 and 3.4 correspondintly).
Now using them emits a deprecation warning.

* Don’t need check_warnings any more.
diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index 7354056..7944cf1 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -1430,6 +1430,7 @@
         self._tail = 1
         return self._last
 
+_sentinel = ['sentinel']
 
 # also see ElementTree and TreeBuilder
 class XMLParser:
@@ -1443,7 +1444,11 @@
 
     """
 
-    def __init__(self, html=0, target=None, encoding=None):
+    def __init__(self, html=_sentinel, target=None, encoding=None):
+        if html is not _sentinel:
+            warnings.warn(
+                "The html argument of XMLParser() is deprecated",
+                DeprecationWarning, stacklevel=2)
         try:
             from xml.parsers import expat
         except ImportError: