closes issue18042 -- a `unique` decorator is added to enum.py

The docs also clarify the 'Interesting Example' duplicate-free enum is for
demonstration purposes.
#18480: Add missing PyType_Ready call to _elementtree extension
(3.3->default): #18480: Add missing PyType_Ready call to _elementtree extension
Closes #18479: Changed venv Activate.ps1 to make deactivate a function, and removed Deactivate.ps1.
Issue #18408: Fix list_ass_slice(), handle list_resize() failure

I tested the patch manually by injecting a fault using gdb: list items are
correctly restored on failure.
Prevent dangling threads/process warning for test_multiprocessing.
Merge.