* Update playlist endpoints to modern format
Deprecate user_playlist_* in favor of the following replacements:
* user_playlist_change_details -> playlist_change_details
* user_playlist_unfollow -> current_user_unfollow_playlist
* user_playlist_add_tracks -> playlist_add_tracks
* user_playlist_replace_tracks -> playlist_replace_tracks
* user_playlist_reorder_tracks -> playlist_reorder_tracks
* user_playlist_remove_all_occurrences_of_tracks -> playlist_remove_all_occurrences_of_tracks
* user_playlist_remove_specific_occurrences_of_tracks -> playlist_remove_specific_occurrences_of_tracks
* user_playlist_follow_playlist -> current_user_follow_playlist
* user_playlist_is_following -> playlist_is_following
* Add current_user_following_artists and current_user_following_users
* Update tests and examples
Resolve TODO in test_user_endpoints.py > SpotifyFollowApiTests.test_user_follows_and_unfollows_user
Use modern playlist endpoints (no username required) in tests and examples.
* Update changelog
* Deprecate playlist_tracks in favor of playlist_items
* Link deprecated functions to new functions and change tracks to items
* Fix references to playlist_tracks
* Change test_playlist_add_items as requested
