to avoid bugging on `pybuf->ob_refcnt != 1`when 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: nexedi/wendelin.core#7

/cc @Tyagov @klaus @jm
/reviewed-on nexedi/wendelin.core!3