make marshal output not dependent on reference count

Bug report

Currently the marshal module will emit a previously-unseen object flagged as a potential reference from later objects, unless the object has a reference count of 1. See https://github.com/python/cpython/blob/b27b57c6e44a276c8a9843fd37d4cf65b2827d5c/Python/marshal.c#L305

This is an overly-conservative heuristic -- it's easy to construct cases where an object has a reference count >1 but is not actually referenced by any other object about to be marshaled, so FLAG_REF is set when it does not need to be.

This makes marshal output unstable depending on accidents of reference counting behavior in the code calling marshal.dumps.

I ran into this because the Cinder JIT is able to reduce unnecessary increfs, and that resulted in some importlib tests failing on comparison of marshal output at https://github.com/python/cpython/blob/b27b57c6e44a276c8a9843fd37d4cf65b2827d5c/Lib/test/test_importlib/test_abc.py#L870-L871 because under Cinder JIT the reference count of code_object in that method is 1.

This previously caused issues in distutils reproducibility, resulting in a partial fix that applies only to interned strings: https://github.com/python/cpython/pull/8226

It would be better if marshal would actually determine which objects have multiple parents in the DAG and deterministically use FLAG_REF or not based on that.

Bắt đầu với Python/marshal.c tại heuristic đếm tham chiếu được đề cập và kiểm tra các assertion bị ảnh hưởng trong Lib/test/test_importlib/test_abc.py ở các dòng 870-871. Theo dõi cách marshal.dumps quyết định FLAG_REF, sau đó xây dựng các kiểm thử cho thấy đầu ra không phụ thuộc vào số lượng tham chiếu của bên gọi và xác nhận các kiểm thử importlib đều chạy qua.