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 degrade performance by 24.72%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 139 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	into_u128_max	1.5 µs	2 µs	-24.72%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ImFeH2:issue-5953 (811d35c) with main (8fcf8fc)}

[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.