fix: release pyclass dict during deallocation

[ImFeH2]

Fixes #5953.

PyClassDictSlot::clear_dict previously called PyDict_Clear, which removed the contents of __dict__ but did not release the pyclass instance's reference to the dictionary itself. This leaked the empty dictionary when the instance was deallocated.

Use Py_CLEAR to clear the slot and decrement the dictionary's reference count.

[codspeed-hq]

Merging this PR will not alter performance

✅ 140 untouched benchmarks

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_{Comparing ImFeH2:issue-5953 (ca796ad) with main (df72d66)}

[ImFeH2]

The reported into_u128_max regression appears unrelated to this change, which only affects #[pyclass(dict)] deallocation. CodSpeed also reports that the comparison used different runtime environments.

[ngoldbaum]

Just a couple minor nits. I agree that this fixes a leak.

[ngoldbaum]

I opened #6234 which fixes a related issue that only happens on Python 3.11 and 3.12, I'd appreciate your eyes on that and the test I came up with there.