Implement keys() and items() for AttrDict

[miguelgrinberg]

Fixes #1535

[pquentin]

Thanks! LGTM.

[kaos]

This breaks uses where the wrapped data has "keys" or "items" in _d_ already.

That is, this used to work, but is now broken:

  a = utils.AttrDict({"items": [1, 2, 3]})
  assert a.items == [1, 2, 3]

[miguelgrinberg]

@kaos Thanks, I have missed this side effect. This limitation has existed in the InnerDoc class for a long time as well:

>>> from elasticsearch_dsl import InnerDoc, Text
>>> class MyDoc(InnerDoc):
...     items = Text()
...
>>> m = MyDoc(items="1,2,3")
>>> m.items
<bound method AttrDict.items of {'items': '1,2,3'}>

The workaround there was to use the dictionary syntax for these reserved names:

>>> m["items"]
'1,2,3'

So this is now also a limitation in AttrDict, and the same workaround applies.