Support inheritance for model option classes

[miracle2k]

This currently is not possible:

class BaseMeta(object):
    app_label = "myapp"

class Company(models.Model):
    class Meta(BaseMeta):
        pass

The reason is that db.models.options.Options.contribute_to_class() uses __dict__ to determine which attributes have been given. It attempts to delete from __dict__, which apparently is not allowed for new-style classes, but __dict__ will not include attributes of superclasses anyway.

I attached a simple patch that uses dir() instead - if been using it for a while and there don't seem to be any problems.

Note that the patch will *not* allow custom meta attributes by prefixing them with __:

metakritik.site: 'class Meta' got invalid attribute(s): _BaseMeta__myopt

[Jacob]

​Fixed in queryset-refactor

[Malcolm Tredinnick]

(In [7477]) Merged the queryset-refactor branch into trunk.

This is a big internal change, but mostly backwards compatible with existing
code. Also adds a couple of new features.

Fixed #245, #1050, #1656, #1801, #2076, #2091, #2150, #2253, #2306, #2400, #2430, #2482, #2496, #2676, #2737, #2874, #2902, #2939, #3037, #3141, #3288, #3440, #3592, #3739, #4088, #4260, #4289, #4306, #4358, #4464, #4510, #4858, #5012, #5020, #5261, #5295, #5321, #5324, #5325, #5555, #5707, #5796, #5817, #5987, #6018, #6074, #6088, #6154, #6177, #6180, #6203, #6658