Concurrent reads from cell objects (LOAD_DEREF) do not scale well

[colesbury]

Concurrent accesses from multiple threads to the same cell object do not scale well in the free-threaded build. There are two problems:

1. LOAD_DEREF calls PyCell_GetRef, which acquires the per-object lock on the cell instance. We'll need to use _Py_TryXGetRef or similar to safely access the value.

2. PyCell_GetRef returns a strong reference and LOAD_DEREF will need a _PyStackRef to handle objects that use deferred reference counting.

Linked PRs

• [WIP] gh-123358: Update LOAD_DEREF to use stackref and atomic incref #124463

[colesbury]

This came up in the context of wjakob/nanobind#695

[corona10]

@colesbury Can I take a look at it?