This is one of a series of bug reports / PRs that lay the groundwork for making pathlib extensible. See here for detail: https://discuss.python.org/t/make-pathlib-extensible/3428

Currently `_Accessor.open()` is expected to function like `os.open()` and return a file descriptor. I'd suggest this interface is too low-level if our eventual aim is to allow users to implement their own accessor.

It would be better is `_Accessor.open()` is expected to function like `io.open()` and return a file object. That way, accessors don't need to deal with file descriptors at all, which is important if they're working with remote filesystems.

I'm planning to wait for bpo-39895 / gh-18838 to land before starting work on this.