Django used to provide a class called SortedDict, which has long been

deprecated in favor of Python's own OrderedDict. However, due to the

way that pickling works, older signatures would still attempt to loading

a SortedDict class.
This change adds a compatibility mechanism for this. Upon finding an

attempt to load a SortedDict, we instead give it a forwarding object

that's compatible with the unpickle code that constructs a new

OrderedDict. It's a bit hacky, in that we need to have this class that

subclasses dict, overrides __new__, and then returns an entirely

different object, but it's a necessity for the compatibility.

Successfully loaded an older signature on Python 3.7/Django 1.11.