bigfile/py: Garbage-collect BigFile <=> BigFileH cycles
For wendelin.core 1 it fixes a small memory leak.
For wendelin.core 2 this is required, because if fileh is left uncollected, WCFS connection associated with that fileh is left unreleased and holding opened files on WCFS filesystem. That in turn prevents WCFS server automatically spawned during the tests from being unmounted and makes tests fail.
/cc @vpelletier