obj.archive_path doesn't exist on ArchivedURL when we create it with open_archive_filepath

[palewire]

Coul there be an exists tests in the __init__ or maybe just allow it to be set automatically when the class is instantiated inside open_archive_filepath.

            path = os.path.join(self.this_dir, "bin/storytracker-archive")
            cmd2 = "%s %s --do-not-compress --output-dir=%s" % (
                path, self.simple_url, self.tmpdir
            )
            code2, path2, err2 = Command(cmd2).run(timeout=3)
            obj2 = storytracker.open_archive_filepath(path2)
            # If you ran obj2.archive_path right here it would return None when the file exists
            self.assertTrue(os.path.exists(path2))
            os.remove(path2)