Parents may be unlinked while another generator sharing part of the
chain is running. As such, we cannot assume that the parent chain
goes all the way to the root. Instead walk backwards from root to
leaf, like we also do during destruction.

Make sure we always update root and parent references before
releasing the old root object.

This fixes some crashes under aggressive cycle collection, so
hopefully this will also fix bug #75351.

Do not run finally blocks in generators on unclean shutdown (e.g.
caused by exit). This is consistent with how finally blocks outside
of generators behave.