bigfile/py: Teach loadblk() to automatically break reference cycles to pybuf
to avoid bugging on pybuf->ob_refcnt != 1when an exception was internally raised & caught somewhere in loadblk() implementation.
Details are in 9aa6a5d7, 61b18a40. The last patch also resorts to buffer unpinning when nothing helps (please see details about unpinning there).
Fixes: #7 (closed)