gh-139852: Add PyObject_GetDict() function

[vstinner]

• Issue: [C API] Add PyObject_GetDict() function #139852

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📚 Documentation preview 📚: https://cpython-previews--139968.org.readthedocs.build/

[vstinner]

The function is similar to PyObject_GenericGetDict() but has a different API and doesn't raise an exception if the __dict__ doesn't exist.

[encukou]

For the docs I propose the wording here: vstinner#11

[vstinner]

I created capi-workgroup/decisions#82 for the C API Working Group.

[vstinner]

I created capi-workgroup/decisions#82 for the C API Working Group.

And the decision is to not add this function. So I close the PR.