Apply most best practices of PEP 8

This refactoring includes:
- Two empty lines before each global function
- Using '{0} {1}'.format(str1, str2) instead of str1 + ' ' + str2
  Sometimes this will make lines longer, sometimes shorter.
- Starting all comments with # + space + comment
- Make lines not longer than 80 characters in most cases
- Renaming some variables to make more sense
- Add some missing code like returns and Exceptions

Not included, but follows:
- Make some comments docstrings
- Rename all 'file' variables, only for Python 2
- Remove some way too verbose comments ;)

test/test_username.py

@@ -4,18 +4,21 @@ import spotdl
 
 username = 'alex'
 
+
 def test_user():
     expect_playlists = 7
     playlists = spotdl.spotify.user_playlists(username)
     playlists = len(playlists['items'])
     assert playlists == expect_playlists
 
+
 def test_playlist():
     expect_tracks = 14
     playlist = spotdl.spotify.user_playlists(username)['items'][0]
     tracks = playlist['tracks']['total']
     assert tracks == expect_tracks
 
+
 def test_tracks():
     playlist = spotdl.spotify.user_playlists(username)['items'][0]
     expect_lines = playlist['tracks']['total']