Similar to the bug in tuple() shown in the current
(r47245) version of Lib/test/crashers/gc_inspection.py,
filter() can be exploited in similar ways.

Rather than the tricky generator used to exploit
tuple(), the attached test case uses a subclass of
tuple with a malicious __getitem__ method. The pattern
being exploited is the same, however: a built-in
function pre-allocates a tuple, then fills it using
calls to user-defined code.

gc_inspection.py.diff also expands the infrastructure
in gc_inspection.py, allowing multiple test functions
to run that could crash the interpreter.

The second patch, fix_filter_crash.patch, is against
Python/bltinmodule.c and adds
_PyObject_GC_TRACK/UNTRACK macros around the call to
the type's sq_item slot in filtertuple().