Support Element truthiness on Python 3

Python 3 changed `__nonzero__` for `__bool__`
(https://portingguide.readthedocs.io/en/latest/core-obj-misc.html#customizing-truthiness-bool)
-- this makes the standard class implementation Python 3 and
provides an alias for Python 2 compatibility.

CHANGELOG.md

@@ -7,6 +7,7 @@ Unreleased
 - dropped support for Python 2.6, 3.3
 - fixed support for Python 3.6 ([#57](https://github.com/stchris/untangle/pull/57))
 - formatted code with black
+- support Element truthiness on Python 3
 
 1.1.1
 - addded generic SAX feature toggle ([#26](https://github.com/stchris/untangle/pull/26))

tests/tests.py

@@ -31,6 +31,13 @@ def test_basic_with_decl(self):
         self.assertTrue("c" in o.a)
         self.assertTrue("d" not in o.a)
 
+    def test_truthiness(self):
+        o = untangle.parse("<a><b/><c/></a>")
+        self.assertTrue(o)
+        self.assertTrue(o.a)
+        self.assertTrue(o.a.b)
+        self.assertTrue(o.a.c)
+
     def test_with_attributes(self):
         o = untangle.parse(
             """

untangle.py

@@ -114,9 +114,11 @@ def __repr__(self):
             self.cdata,
         )
 
-    def __nonzero__(self):
+    def __bool__(self):
         return self.is_root or self._name is not None
 
+    __nonzero__ = __bool__
+
     def __eq__(self, val):
         return self.cdata == val