Check Support for Calibre 5

[fallaciousreasoning]

Calibre 5 migrated from Python 2.7 to Python 3. I'm unsure whether this plugin still works.

[harmtemolder]

Checking it now, will update when needed

[harmtemolder]

I upgraded everything needed for Python 3 (and did some code cleanup). @fallaciousreasoning Could you test v0.4?

[fallaciousreasoning]

Awesome nice work! It all seems to be working :)