In python < 3.6, fields order is not preserved in generated init

[smarie]

We should probably use a creation_counter on fields like django did:

https://github.com/django/django/blob/129583a0d3cf69b08d058cd751d777588801b7ad/django/db/models/fields/__init__.py#L96

https://github.com/django/django/blob/f3c43ad1fd9556f0fd026a5dfa93c67a5cf186ca/django/forms/forms.py#L38

[smarie]

Note to future self: this can be solved using metaclasses in python 3.x < 3.6 but not in python 2 since the __prepare__ method did not exist at the time;

class OrderTrackingMeta(type):
    """
    Metaclass to track the order of class members.
    The problem with this is that it is not compliant with python 2.
    """
    # The prepare function
    @classmethod
    def __prepare__(metacls, name, bases):  # No keywords in this case
        return OrderedDict()

    # The metaclass invocation
    def __new__(cls, name, bases, classdict):
        new_type = type.__new__(cls, name, bases, classdict)
        new_type.__ordered_dict__ = classdict
        return new_type

See https://www.python.org/dev/peps/pep-3115/#example and https://stackoverflow.com/a/27113652/7262247