bpo-40696: Fix a hang that can arise after gen.throw() (GH-20287)

bpo-40696: Fix a hang that can arise after gen.throw() (GH-20287)

This updates _PyErr_ChainStackItem() to use _PyErr_SetObject()
instead of _PyErr_ChainExceptions(). This prevents a hang in
certain circumstances because _PyErr_SetObject() performs checks
to prevent cycles in the exception context chain while
_PyErr_ChainExceptions() doesn't.
(cherry picked from commit 7c30d12bd5359b0f66c4fbc98aa055398bcc8a7e)

Co-authored-by: Chris Jerdonek <chris.jerdonek@gmail.com>