Advanced examples (#986)

* Created a testing suite for the spotipy client.Spotify class

* Delete pyvenv.cfg

* Corrected error in description

* Added some advanced usage examples, some of which involve utilizing basic data analysis methods using other libraries such as Pandas

* Minor formatting fixes

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Co-authored-by: Stéphane Bruckert <stephane.bruckert@gmail.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ Add your changes below.
 - Added test_artist_id, test_artist_url, and test_artists_mixed_ids to non_user_endpoints test.py
 - Added rate/request limit to FAQ
 - Added custom `urllib3.Retry` class for printing a warning when a rate/request limit is reached.
+- Added `personalized_playlist.py`, `track_recommendations.py`, and `audio_features_analysis.py` to `/examples`.
 
 ### Fixed
 - Audiobook integration tests