Add docstrings; Remove verbose comments; Fix errors introduced with cleanup

This comment will:
- Transform docstrings above functions into docstrings
- Remove some way too verbose comments
- Apply some more recommendations from PEP8 forgotten last time
- Fix some errors introduced with the first code cleanup

Work left to do:
- Add params to docstrings
- Rename file variables

test/test_username.py

@@ -22,7 +22,8 @@ def test_playlist():
 def test_tracks():
     playlist = spotdl.spotify.user_playlists(username)['items'][0]
     expect_lines = playlist['tracks']['total']
-    result = spotdl.spotify.user_playlist(playlist['owner']['id'], playlist['id'], fields='tracks,next')
+    result = spotdl.spotify.user_playlist(
+        playlist['owner']['id'], playlist['id'], fields='tracks,next')
     tracks = result['tracks']
 
     with open('list.txt', 'a') as fout: