Release v0.5.1

Fixed

• build_auth_file() no longer raises an AttributeError when its filename is passed as a str, which its signature allows. The failure happened after the login had already registered the device, so the registration was lost without the auth file being written (#273)
• The convert-oa-file plugin works again. It has failed to even load since v0.2 because it imported pass_session from the wrong module, and behind that sat a tuple passed to pathlib.Path(), a call to a Session method that does not exist, an expiry read as seconds although OpenAudible writes milliseconds, a KeyError on cookie fields that are optional, and a profile name from the converted file that could write outside the config directory (#286)
• The size of a finished audiobook download is checked again. The check existed but nothing ever called it, so a file that arrived at a different length than announced was renamed to its final name and counted as a success. Covers and PDFs take the older download path and were never affected (#291)