Type annotate as IO[bytes], not BinaryIO

[hukkin]

Fixes #214

I'm currently not creating a SupportsRead because Protocols only appear in Python 3.8 and we need to support 3.7 (and I don't want a typing-extensions dependency).

[hauntsaninja]

Great! This should fix the user's issue.

Note that you don't necessarily have to have a runtime dependency on typing_extensions. Type checkers all special case typing_extensions, so they'll still recognise the following even if typing_extensions is not installed:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing_extensions import Protocol

    class SupportsReadBytes(Protocol):
        def read(self) -> bytes: ...
else:
    SupportsReadBytes = object

That said, it is pretty gross and it looks like this hasn't come up much, so fine to keep PR as is.

[hukkin]

Thanks Shantanu for your insight on this one!

Yeah, I agree it's a bit gross so let's try to avoid it until a user complains. Hopefully that never happens, as more and more users move to new Python versions and tomllib 😃

[mgorny]

That said, i wish some basic protocols like SupportsReadBytes were part of stdlib. It feels like it's going to end up being repeated a lot.