toplist: Add Toplist.error
toplist: Add Toplist.backend_request_duration
toplist: Add Toplist.tracks
toplist: Add Toplist.albums
toplist: Add Toplist.artists
docs: Update development status
toplist: Check if None instead of falsy
toplist: Add repr(toplist)
toplist: Fix Python 3 test breakage
toplist: Add example
inbox: Add missing test teardown
toplist: Add missing test teardown and setup
tests: Make specific test files runnable
album: Add AlbumBrowser
album: Add AlbumBrowser.album and repr()
album: Add AlbumBrowser.is_loaded and .load()
album: Add AlbumBrowser.error
album: Add AlbumBrowser.backend_request_duration
docs: Refer to correct PEP
album: Skip the flaky test

...instead of the test that makes it flaky, since it looks like other tests
may make this test flaky as well.