Fix artist.get_bio_content() when bio is empty + other test fixes

[hugovk]

Changes proposed in this pull request:

Code fixes:

• Fix artist.get_bio_content() for when bio is empty: previously Last.fm returned <content/> (Fix artist.get_bio_content() to return None if bio is empty #326), but now it doesn't even return a <content/> section. See below.

Test fixes:

• xfail test_track_mbid: Last.fm is broken and returning "Track not found"
  • https://support.last.fm/t/track-getinfo-with-mbid-returns-6-track-not-found/47905
• Fix test_get_cover_image: the test image is now gif not png
• Fix test_artist_mbid: check more than one title

Test updates:

• Update pre-commit and add quarterly autoupdate_schedule

---

Compare with #326:

{
"artist": {
"ontour": 0,
"stats": {
"playcount": 8,
"listeners": 6
},
"mbid": "",
"image": [
{
"size": "small",
"#text": "https://lastfm.freetls.fastly.net/i/u/34s/2a96cbd8b46e442fc41c2b86b821562f.png"
},
{
"size": "medium",
"#text": "https://lastfm.freetls.fastly.net/i/u/64s/2a96cbd8b46e442fc41c2b86b821562f.png"
},
{
"size": "large",
"#text": "https://lastfm.freetls.fastly.net/i/u/174s/2a96cbd8b46e442fc41c2b86b821562f.png"
},
{
"size": "extralarge",
"#text": "https://lastfm.freetls.fastly.net/i/u/300x300/2a96cbd8b46e442fc41c2b86b821562f.png"
},
{
"size": "mega",
"#text": "https://lastfm.freetls.fastly.net/i/u/300x300/2a96cbd8b46e442fc41c2b86b821562f.png"
},
{
"size": "",
"#text": "https://lastfm.freetls.fastly.net/i/u/300x300/2a96cbd8b46e442fc41c2b86b821562f.png"
}
],
"similar": "",
"streamable": "0",
"tags": "",
"name": "Mr Sizef   Unquote",
"url": "https://www.last.fm/music/Mr+Sizef+++Unquote"
}
}

[hugovk]

Some race issue with test_love/test_unlove, not going to worry about it for now.