Add type hints to _binary

[nulano]

Helps #2625.

There are two tricky parts in adding type hints to PIL._binary:

The i16* and i32* give a "Returning Any from function declared to return int" in --strict mode due to the use of struct.unpack_from. This can be properly silenced with #type: ignore[no-any-return], but that gives a "Unused type ignore comment" when running without --strict, so I have left it out for now.

The i8 function distinguishes between int and bytes inputs in a way that mypy doesn't recognize. Since i8 is called with an int input only from IptcImagePlugin, I have removed those calls there and removed int support as input to i8.

Although the values in self.info[(3, 60)] and self.info[(3, 65)] can be lists if the tags are duplicated, these tags are marked as "non-repeatable" in the specification (pg. 13-14). The specification recommends using the first entry if the tags are duplicated (sec. 1.5-d), but this recommendation was already ignored in IptcImagePlugin and I was unable to find any images to test this with.

In doing so, I found that IptcImagePlugin.dump, IptcImagePlugin.i, and IptcImagePlugin.PAD are simple helpers that should probably be deprecated (edit: #7664).

[hugovk]

This looks fine, please can you split this into two PRs, one to deprecate and the other to add type hints?

I think it will be clearer to keep them separate.

If you want, it's okay to leave this PR as-is, and create another one that is the first step, and we can merge that one first.

[nulano]

OK, I've removed the deprecation commits and created #7664 for that.

[hugovk]

Thanks!