Add ability to access dict entries

[twidi]

This works this way:

The solver starts by trying to access the name as an attribute.
If an AttributeError exception is raised, then it tries to access it
via a dict-like approach.
If it still fail via a KeyError exception, then a
AttributeNotFound excetption is raised.

The key must of course be one of the attribute allowed for the class.

Their is no way to specify keys to retrieve for a simple dict. If the
dict class is added as a source to the registry allowing all
attributes, then all keys will be allowed for all dicts (if the dict class is not registered as a source, no key will be available for simple dicts)

It still possible, though, to use a function at the upper level that
will act as an attribute and will return only some keys we want.

If we have this:

class Klass:
    def func(self):
        return {'foo': 0, 'bar': 1, 'baz': 2}

registry.register(dict)
registry.register(Klass)

Then the user will be able to access func, then foo bar and baz
from the dict.

If we don't want the user to access baz without modifying the class,
we can do this:

def func(obj):
    d = obj.func()
    allowed = {'foo', 'bar'}
    return {k: v for k, v in d.items() if k in allowed}

registry.register(dict)
registry.register(klass, ('func', func))

Then, when solving the func attribute, it's the func function what
will be called, itself calling the method then returning a dict with
only the wanted keys

Another way to do it is:

class FuncDict(dict): pass

def func(obj):
    return FuncDict(obj.func())

registry.register(FuncDict, ['foo', 'bar'])
registry.register(klass, ('func', func))