gh-105927: _ssl GET_SOCKET() uses _PyWeakref_GET_REF()

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• Issue: C API: Add PyWeakref_GetRef() function #105927

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I started hacking _ssl.c to use a strong reference, but the code is more complicated than what I expected. GET_SOCKET() returns NULL if there is no socket, None is the weak reference is dead, or a borrowed reference to the socket otherwise. It's not easy to migrate the code to strong references. So I only changed GET_SOCKET() but it still returns a borrowed reference.