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