array.array should support `__class_getitem__`

[JelleZijlstra]

The array.array class has been marked as generic in typeshed since 2017 (python/typeshed#1649). We just got a report on mypy (python/mypy#13942) pointing out that the class is not subscriptable at runtime. Just like other generic standard library classes, we should make array.array subscriptable at runtime (PEP-585).

I'll make a PR.

[serhiy-storchaka]

Isn't it a bug in typeshed? This class does not look generic.

[JelleZijlstra]

It's marked as generic because the "element" type depends on the type code:

In [50]: a1=array.array('b', [1])

In [51]: a1[0]
Out[51]: 1

In [52]: a2 = array.array('f', [1])

In [53]: a2[0]
Out[53]: 1.0

So we'd want a1 to be typed as an array[int] and a2 as an array[float].

[serhiy-storchaka]

It makes sense.