[WIP] gh-85283: Build _csv extension with limited C API

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• Replace PyObject_VectorcallDict() with PyObject_Call() in _call_dialect(). Create a temporary tuple to pass the positional dialect argument.
• Use PyUnicode_DecodeUTF32() to create an Unicode string from Py_UCS4 characters.
• Replace PyErr_Format("%.200s", Py_TYPE(src)->tp_name) with PyErr_Format("%T", src).
• Replace PyUnicode_READ_CHAR() with PyUnicode_ReadChar().
• Replace PyUnicode_READ() with PyUnicode_ReadChar(). join_append_lineterminator() now takes an object (Unicode string) as argument.
• Replace PyUnicode_GET_LENGTH() with PyUnicode_GetLength().
• Replace Py_XSETREF() with assignement and Py_XDECREF().
• Replace PyDict_Pop() with PyDict_DelItem().
• Use PyType_GetSlot() to get tp_clear and tp_alloc slots.
• Add some casts to (PyObject*).
• Add an explicit #include <string.h>.

• Issue: [C API] Convert a few stdlib extensions to the limited C API (PEP 384) #85283

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I don't think that it's worth it. I prefer to close this PR.