[Python-Dev] Check dict implementation details

[Serhiy Storchaka]

Since dict is ordered in CPython 3.6, it can be used instead of 
OrderedDict in some places (e.g. for implementing simple limited 
caches). But since this is implementation detail, it can't be used in 
the stdlib unconditionally. Needed a way to check whether dict is ordered.

Actually there are two levels of "ordering".

1. Dict without deletions is iterated in the order of adding items. 
Raymond's original compact dict implementation satisfied this claim.

2. In addition the order is preserved after deletion operations. Naoki's 
implementation satisfies this more strong claim.