Memory leak on `dict` pyclasses after setting dynamic attribute

[threecgreen]

Bug Description

I've created a repo with a single pyclass(dict) struct exported to Python. I see linear memory growth with tracemalloc after setattring a new dynamic field on each object. With the same object, if I don't create an attribute (and the dict is never created), the memory is freed as expected.

In clear_dict it looks like the dict is cleared but the ref count isn't decremented.

Steps to Reproduce

1. git clone https://github.com/threecgreen/pyo3-dict-leak-repro
2. cd pyo3-dict-leak-repro
3. pip install maturin && maturin develop
4. python test_leak.py

Backtrace

Without setattr:         1,140 bytes  (0.0 B/obj)
With setattr:       12,801,140 bytes  (64.0 B/obj)
Delta:              12,800,000 bytes
Expected: ~0 bytes leaked in both cases.
Actual:   ~64 bytes leaked per object when __dict__ is populated.

Your operating system and version

Linux

Your Python version (python --version)

3.10.10

Your Rust version (rustc --version)

1.94.0

Your PyO3 version

0.28.3

How did you install python? Did you use a virtualenv?

pyenv

Additional Info

No response