Add new features from CPython (finalizer, close, "with" support, dir_fd handling)

[benhoyt]

Since the last changes to this scandir module, CPython has fixed a couple of issues like adding a finalizer and adding a .close() method to the iterator. Python 3.6 added support for the with statement too. These changes from CPython should be included here.

[benhoyt]

I don't really have much time to work on scandir at this point, but if someone wants to submit a PR to update the library to include these features from CPython, go ahead!