Metaclasses
-----------

Python 3 and Python 2 syntax for metaclasses are incompatible.
``future`` provides a function (from ``jinja2/_compat.py``) called
:func:`with_metaclass` that can assist with specifying metaclasses
portably across Py3 and Py2. Use it like this::

    from future.utils import with_metaclass

    class BaseForm(object):
        pass

    class FormType(type):
        pass

    class Form(with_metaclass(FormType, BaseForm)):
        pass