2.21: Flask cache handler

Added

• Added market parameter to album and albums to address #753 by @ivyadam
• Added 'show_featured_artists.py' to 'examples'.
• Expanded contribution and license sections of the documentation.
• Added FlaskSessionCacheHandler, a cache handler that stores the token info in a flask session.
• Added Python 3.10 in GitHub Actions

Fixed

• Updated the documentation to specify ISO-639-1 language codes.
• Fix AttributeError for text attribute of the Response object, #811 by @rtcq
• Require redis v3 if python2.7 (fixes readthedocs)