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

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):

        __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.