Creates a new class derived from ModelBase.

def create_model_class(cls, name, table_name, attrs, validations=None, *, otherattrs=None,
                         other_bases=None):
    """Creates a new class derived from ModelBase."""
    members = dict(table_name=table_name, attrs=attrs, validations=validations or [])
    if otherattrs:
      members.update(otherattrs)
    return type(name, other_bases or (), members)