Fix published_date parsing

[NolanT]

Fixing issue noted in #58
Libby is now using "%Y-%m-%dT%H:%M:%S%z format for the published_date. This needs to be converted to %Y-%M-%D to be added to ID3 tags.

Additionally, fixed mypy union-attr errors

[coveralls]

coverage: 88.456% (-0.1%) from 88.561%
when pulling c5ea1a2 on NolanT:patch-1
into 40f9be2 on ping:master.