Fixes for Python 3.10

[hypsometric]

Hi, I've been testing the latest develop branches with Python 3.10 (not sure which version is the target) For now, I had to make the following changes to make things work with my config:

• deluge: b64encode returns bytes, transform into string before serializing to JSON.
• deluge: no more need to try different encodings for the torrent filename or the base64-encoded content, should all be unicode strings with Python 3
• collections.abc: Iterable, Mapping, MutableMapping moved from collections to collections.abc since Python 3.3
• pygazelle: html.parser.HTMLParser().unescape() moved to html.unescape() in Python 3.4

[rembo10]

Hey thanks for this, I didn't test any of the deluge stuff... So it's really helpful. Not sure why apscheduler and mutagen were working ok for me. I think I might try to update those libraries.