Spurious typecheck failure when passing Gzipfile instance to tarfile.open

[ncoghlan]

Due to #2580, the following code reports a typechecking error:

# Storage creation is conditional in the real code, hence `... | None` in the error message
storage = GzipFile(archive_name, mode="w", mtime=0)
tar = tarfile.open(archive_name, tar_mode, fileobj=storage)

error: Argument "fileobj" to "open" has incompatible type "GzipFile | None"; expected "IO[bytes] | None"  [arg-type]

The comments on #2580 suggested that tarfile.open should be migrated over to protocol checking instead, but I'm not sure what that actually looks like for IO[bytes].

[srittau]

It seems that we already have a protocol _Fileobj, but we missed to replace a few instance in the open() annotations.