PyType_FromSpec may leak memory with repeated slot IDs

[encukou]

If a slot ID is repeated in PyType_Spec, the current PyType_FromSpec implementation happens to usually use the last one and ignore earlier ones, but:

• if Py_tp_doc is repeated, PyType_FromSpec may leak memory, and
• if Py_tp_members is repeated, PyType_FromSpec may memcpy unintialized memory (if slot specifies fewer members than the last one).

We chould:

• clarify in the docs that repeated slot IDs are not supported
• raise an exception for duplicate slots (at least when it's easy to detect, as in the cases above)

Alternatively, we could formalize that the last slot of a given ID “wins” and fix the leak/mem issue, but I don't see a use case for this more complex behavior.