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@Haypo @serhiy-storchaka @methane please review this PR

Before review, why did you added test for private utility functions?
They are not covered other tests for higher level functions? Do you want cover
edge case of them which cannot test from higher level functions?

@methane, because of edge cases. Also it seems easier to write such whitebox tests for small isolated units(and this is what documentation suggests if I understood correctly). Generally I want to write tests for the whole module and because there will be quite a few tests I decided to split it into multiple small PRs.

OK. I leave some comments.

But I think I'm not suited to this change, because this code uses some inspect magic and
I'm not familiar with inspect and pdb module.
I don't know is there any better way other than than proposed magic.

@methane thank you very much for comments, can you please suggest person who can review test for pdb?

I usually use Experts Index and git history to find experts.

Experts Index doesn't tell expert of pdb. But @1st1 is listed as expert of inspect module.
As reading history of pdb, @birkenfeld did very important jobs for pdb.

And there are some comments in b.p.o

While pdb and inspect relying on implementation, PyPy is compatible at this point.
So I don't think cpython_only is required for this, until Python 3.6 implementation
which has incompatible code object born.