Message 258724 - Python tracker

Patch version 4:

Fix _PyCode_ConstantKey:

* always return a tuple
* create a frozenset/set if input is a set/frozenset, items are unordered
* enhance unit tests: compare values using repr() to compare value + type at once
* fix reference leak

Other minor changes to take Serhiy's comments in account.

Thanks Serhiy for good suggestions!