[3.6] bpo-31455: Fix an assertion failure in ElementTree.XMLParser(). (GH-3545) (#3585)

* Avoid calling "PyObject_GetAttrString()" (and potentially executing user code) with a live exception set.

* Ignore only AttributeError on attribute lookups in ElementTree.XMLParser() and propagate all other exceptions.
(cherry picked from commit c8d8e15bfc)

Lib/test/test_xml_etree.py

@@ -2476,6 +2476,31 @@ class TreeBuilderTest(unittest.TestCase):
             ('html', '-//W3C//DTD XHTML 1.0 Transitional//EN',
              'http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd'))
 
+    def test_builder_lookup_errors(self):
+        class RaisingBuilder:
+            def __init__(self, raise_in=None, what=ValueError):
+                self.raise_in = raise_in
+                self.what = what
+
+            def __getattr__(self, name):
+                if name == self.raise_in:
+                    raise self.what(self.raise_in)
+                def handle(*args):
+                    pass
+                return handle
+
+        ET.XMLParser(target=RaisingBuilder())
+        # cET also checks for 'close' and 'doctype', PyET does it only at need
+        for event in ('start', 'data', 'end', 'comment', 'pi'):
+            with self.assertRaisesRegex(ValueError, event):
+                ET.XMLParser(target=RaisingBuilder(event))
+
+        ET.XMLParser(target=RaisingBuilder(what=AttributeError))
+        for event in ('start', 'data', 'end', 'comment', 'pi'):
+            parser = ET.XMLParser(target=RaisingBuilder(event, what=AttributeError))
+            parser.feed(self.sample1)
+            self.assertIsNone(parser.close())
+
 
 class XMLParserTest(unittest.TestCase):
     sample1 = b'<file><line>22</line></file>'