(I prefer to not split the discussion on this issue and my 2 PR, I prefer to only discuss the design in this issue.)

https://github.com/python/cpython/pull/3927#pullrequestreview-67963845

Serhiy:
> Using the requires_type_collecting decorator LGTM. But I don't sure about adding PYTHONSHOWALLOCCOUNT. This looks like a new feature to me.
> What if split this PR on two parts? The one fixes issues related to immortal types, the other is about additional output.

Both changes are connected. If you only want to fix tests, that's the purpose of my much longer PR 3910.

"This looks like a new feature to me."

My PR changes the default behaviour, but COUNT_ALLOCS is not enabled by default, not even by the debug mode. You have to explicitely enable it. I expect that developers who use this option are able to track the Python 2.7 changelog and "discover" the newly added environment variable by themself. Moreover, defining PYTHONSHOWALLOCCOUNT=1 is "backward compatible" with Python < 2.7.15 and Python 3, since unknown environment variables are just ignored.