Fix context handling, stale test dirs, and resource leak

[jmgurney]

Sorry, this combines a few fixes all in one, mainly I was dealing w/ a number of them and did a little bit of clean up at the same time.

1. it drops py2 compatibility.
2. it splits _defer_close to not do double duty as closing the file handle, and makes the later a _doclose attribute.
3. adds a test for context management to make sure things are closed and fix it so they work.
4. Fixes SeekableArchive to not leak an open file, this silences a bunch of ResourceWarning: unclosed file when running the tests.
5. Adds support for passing in pathlib.Path.
6. Fixes tests so they clean up the temp directory after they run.
7. Adds test to verify that context doesn't close a passed in file that was opened.

[cabarnes]

You could use TemporaryDirectory and its member cleanup() instead to avoid importing shutil and manually deleting it