Support 'on'/'off' for type ignores

Sometimes it's impossible to put # type: ignore on the offending line - e.g. because it's a multiline string, and there's nowhere you can put the comment outside the string that mypy will recognise. Or more generally, there might be a whole block of code for which mypy has a bunch of spurious complaints, so putting in dozens of # type: ignores is a bit noisy.

The only other option today is to have mypy ignore the entire file, but that has nasty ramifications - beyond just killing type checking for what might be 99% checkable, it also makes any imports of that file invalid, and so you have to spread ignore directives out like a messy spiderweb.

Thus, it'd be great if one could do e.g. # type: ignore on and # type: ignore off, so that the scope could be controlled precisely.

L’issue non indica file, test o punti di ingresso. Inizia tracciando come mypy analizza e applica attualmente le direttive # type: ignore, quindi determina come on e off debbano definire l’ambito delle diagnosi nelle stringhe multilinea e nei blocchi di codice. Il lavoro è completato quando gli ignore con ambito funzionano senza disabilitare il controllo per il resto del file né invalidare gli import.