Add support for "serialization" of deps to native Python objects

[pombredanne]

Several objects and the objects in deps.py in particular are plain (small) objects and they do not have the equivalent of a namedtuple._asdict() or an attrs .asdict() method or similar to return plain collections and/or built-in types for easier integration such as for instance serializing to JSON. This would be a great addition. In contrast most of the deb822 objects have a mapping-like behavior (though that does not make them JSON-serializable either by default). And since they are collections.Mapping they are easier to convert at once to actual built-in types.

[pombredanne]

For the sake of illustration here is a very simple external function that converts deps objects to Python collections objects. It would much simpler to have this transform built-in the deps objects IMHO.

def deps_as_mapping(rel):
    """
    Return a native Python collection from a `rel` deb_pkg_tools.deps.Relationship
    object where every object is converted recursively  to basic collections types e.g. 
    an OrderedDict or a list. Other types are left untouched.
    """
    if isinstance(rel, (deps.RelationshipSet, deps.AlternativeRelationship,)):
        return map(deps_as_mapping, rel.relationships)

    if isinstance(rel, deps.VersionedRelationship):
        return OrderedDict([
            ('name', rel.name),
            ('version', rel.version),
            ('operator', rel.operator),
        ])

    # comes last as this is the least specialized type and the root of the
    # inheritance hierarchy and they have no version and operator
    # attribute therefore we return None for these for consistency.
    if isinstance(rel, deps.Relationship):
        return OrderedDict([
            ('name', rel.name),
            ('version', None),
            ('operator', None),
        ])
    return rel