Unfortunately this patch doesn't fix issue19021. Popen.__del__ is called when whiping the idlelib.rpc module which was collected in a weaklist. But the builtins module also was collected in a weaklist and wiped before idlelib.rpc.

Here is revised patch which fixes issue19021. It defers wiping of the builtins and sys modules (this partially restores 3.3 algorithm) and fixes other related issues (some of these fixes should be backported to older versions).