Document FrameLocalsProxy behaviour quirks

Following PEP 667, https://docs.python.org/dev/reference/datamodel.html#frame-objects mentions that frame.f_locals may be a write-through proxy object, but doesn't really explain what that means.

Once https://github.com/python/peps/pull/3845 lands PEP 667 will contain such a description, but the main docs should at least cover the key behaviours:

• new instance on every attribute access
• two proxies with the same keys and values will still be unequal if they refer to different frames
• extra keys can be stored and are stored on the frame itself so they're shared across all proxies for the same frame
• keys corresponding to local variables can't be deleted
• .copy() (and any other API that produces a new container) returns a regular dict instance

Suggested location would be as a new 4th subsection under the frame objects section linked above. A "see also" from that new subsection back to PEP 667 would also be a useful addition.

Inizia con la sezione sugli oggetti frame nella documentazione del modello dati Python e rivedi PEP 667, inclusa la pull request di PEP collegata. Aggiungi una quarta sottosezione che descriva i cinque comportamenti elencati di FrameLocalsProxy e un link «vedi anche» a PEP 667. Il lavoro è completato quando la documentazione principale spiega l’identità del proxy, le chiavi aggiuntive condivise, le variabili locali non eliminabili e le API che producono dizionari.