Python GC destroys PyGObject with external references

PyGObject traverses its closures in tp_traverse, but the lifetime of the closures
is tied to the lifetime of the GObject and not the wrapper. This confuses
the Python GC when it sees a ref cycle and tries to break it up with tp_clear.
Since tp_clear will not invalidate the closure and only invalidate the Python
wrapper the closure callback gets called with the now cleared/invalid object.

This bug make python gtk programs to segfault randomly.

Patch available here:
https://bugzilla.gnome.org/show_bug.cgi?id=731501

Should apply to 3.24 or be easily backported: one line patch adding a if statement.