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There's no evidence of a leak here -- quite the contrary. 
As the docs say, DEBUG_LEAK implies DEBUG_SAVEALL, and
DEBUG_SAVEALL results in  _all_ cyclic trash getting
appended to gc.garbage.  If you don't mess with
gc.set_debug(), you'll discover that gc.garbage is empty at
the end.

In addition, note that the DEBUG_LEAK output plainly says:

gc: collectable ...

That's also telling you that it found collectable cyclic
trash (which it would have reclaimed had you not forced it
to get appended to gc.garbage instead).  If gc had found
uncollectable cycles, these msgs would have started with

gc: uncollectable ...

instead.

Most directly, if I run your tarfile open() and file
extraction in an infinite loop (without messing with
gc.set_debug()), the process memory use does not grow over time.

Unless you have other evidence of an actual leak, this
report should be closed without action.  Yes, there are
reference cycles here, but they're of kinds cyclic gc reclaims.