Implement locked iteration for PyList (#4789)

* implement locked iteration for PyList

* fix limited API and PyPy support

* fix formatting of safety docstrings

* only define fold and rfold on not(feature = "nightly")

* add missing try_fold implementation on nightly

* Use split borrows for locked iteration for PyList

Inline ListIterImpl implementations by using split borrows and
destructuring let Self { .. } = self destructuring inside
BoundListIterator impls.

Signed-off-by: Manos Pitsidianakis <[email protected]>

* use a function to do the split borrow

* add changelog entries

* fix clippy on limited API and PyPy

* use a macro for the split borrow

* add a test that mutates the list during a fold

* enable next_unchecked on PyPy

* fix incorrect docstring for locked_for_each

* simplify borrows by adding BoundListIterator::with_critical_section

* fix build on GIL-enabled and limited API builds

* fix docs build on MSRV

---------

Signed-off-by: Manos Pitsidianakis <[email protected]>
Co-authored-by: Manos Pitsidianakis <[email protected]>

Author: 2 people

newsfragments/4789.added.md

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+* Added `PyList::locked_for_each`, which is equivalent to `PyList::for_each` on
+  the GIL-enabled build and uses a critical section to lock the list on the
+  free-threaded build, similar to `PyDict::locked_for_each`.

newsfragments/4789.changed.md

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+* Operations that process a PyList via an iterator now use a critical section
+  on the free-threaded build to amortize synchronization cost and prevent race conditions.