[2.7] bpo-33297: Mention Pillow to work with more image formats. (GH-6505) (GH-6562)

Also update PIL doc references to Pillow.
(cherry picked from commit 'b81ca28b378c8b29204a37f8bd433a3379f53f7d')

Co-authored-by: 'Andrés Delfino.34587441+andresdelfino@users.noreply.github.com'
3 files changed