Fixed #19539 -- Updated custom model fields example for Python 3.

Thanks astorije@ for the report.

docs/howto/custom-model-fields.txt

@@ -249,7 +249,7 @@ appropriate Python object. The details of how this happens internally are a
 little complex, but the code you need to write in your ``Field`` class is
 simple: make sure your field subclass uses a special metaclass:
 
-For example::
+For example, on Python 2::
 
     class HandField(models.Field):
 
@@ -258,7 +258,21 @@ For example::
         __metaclass__ = models.SubfieldBase
 
         def __init__(self, *args, **kwargs):
-            # ...
+            ...
+
+On Python 3, in lieu of setting the ``__metaclass__`` attribute, add
+``metaclass`` to the class definition::
+
+    class HandField(models.Field, metaclass=models.SubfieldBase):
+        ...
+
+If you want your code to work on Python 2 & 3, you can use
+:func:`six.with_metaclass`::
+
+    from django.utils.six import with_metaclass
+
+    class HandField(with_metaclass(models.SubfieldBase, models.Field)):
+        ...
 
 This ensures that the :meth:`.to_python` method, documented below, will always
 be called when the attribute is initialized.