It turns out some code wants to store tracebacks e.g. for further
logging/whatever. This way GC won't help to free up references to pybuf.
However if pybuf remain referenced only from calling frames, we can
change there reference to pybuf to a stub object "<pybuf>" and this way
remove the reference.

With added test but without loadblk changes the failure would be as:

    pybigfile_loadblk WARN: pybuf->ob_refcnt != 1 even after GC:
    pybuf (ob_refcnt=2):    <read-write buffer ptr 0x7fae4911f000, size 2097152 at 0x7fae4998cef0>
    pybuf referrers:        [<frame object at 0x556daff41aa0>]		<-- NOTE
    bigfile/_bigfile.c:613 pybigfile_loadblk        BUG!