We now let pickle rebuild as usual the basic Python types (integers,
strings, int, lists, tuples, dictionaries, ...), and convert them to
Python in a post processing stage.

Exported with:

    sage: from sagetypes import Exporter
    sage: e = Exporter()
    sage: e.harvest_categories()
    sage: e.harvest_sage_object(TransitiveGroups())
    sage: for P in [ TransitiveGroup(4,1), Partitions(100), QQ['x,y'] ]:
    ....:     e.harvest_sage_object(P)
    ....:     e.harvest_sage_object(P.an_element())
    sage: e.save('transitive_groups.json')