Remove redundant check from arraymodule b_getitem (GH-14676)

The `arraymodule`'s `b_getitem` function returns a `PyLong` converted 
from `arrayobject`'s array, by dereferencing a pointer to `char`.

When the `char` type is `signed char`, the `if (x >= 128) x -= 256;` comparison/code is redundant because a `signed char` will have a value of `[-128, 127]` and so `x` will never be greater or equal than 128.

This check was indeed needed for situations where a given compiler would assume `char` being `unsigned char` which would make `x` in `[0, 256]` range.

However, the check can be removed if we cast the `ob_item` into a signed char pointer (`signed char*`) instead of `char*`.

This PR/commit introduces this change.