gh-114944: Fix race between `_PyParkingLot_Park` and `_PyParkingLot_UnparkAll` when...

gh-114944: Fix race between `_PyParkingLot_Park` and `_PyParkingLot_UnparkAll` when handling interrupts (#114945)

Fix race between `_PyParkingLot_Park` and `_PyParkingLot_UnparkAll` when handling interrupts

There is a potential race when `_PyParkingLot_UnparkAll` is executing in
one thread and another thread is unblocked because of an interrupt in
`_PyParkingLot_Park`. Consider the following scenario:

1. Thread T0 is blocked[^1] in `_PyParkingLot_Park` on address `A`.
2. Thread T1 executes `_PyParkingLot_UnparkAll` on address `A`. It
   finds the `wait_entry` for `T0` and unlinks[^2] its list node.
3. Immediately after (2), T0 is woken up due to an interrupt. It
   then segfaults trying to unlink[^3] the node that was previously
   unlinked in (2).

To fix this we mark each waiter as unparking before releasing the bucket
lock. `_PyParkingLot_Park` will wait to handle the coming wakeup, and not
attempt to unlink the node, when this field is set. `_PyParkingLot_Unpark`
does this already, presumably to handle this case.