Define a concurrency model (aka memory model) for Python

Over the years there have been attempts to define a memory model, notably PEP 583 and many questions both on the bug tracker and here about expected semantics and atomicity of operations, e.g. What is Thread Safety as applied to Python? and What is the Long-Term Vision for a Parallel Python Programming Model? Despite that, there is AFAICT, nothing in the docs. We have an object model but there is no mention of concurrent access. We should write down concurrency models for both with-GIL and fr...