Alexey, even though the refleak does not appear to be related to your earlier PR, you are welcome to keep working on this issue. :)  If you do then please add me as a reviewer on whatever PR you make.  Also, I'd be glad to answer any questions you have about the _xxsubinterpreters module.  The more folks that understand that code, the better. :)

On the other hand, if you decide at this point that you'd rather do something else than track down refleaks I introduced then I'd totally understand. :)  In that case feel free to assign this issue to me at that time.