[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'
diff --git a/Doc/library/tkinter.rst b/Doc/library/tkinter.rst
index 7d80b40..5e07051 100644
--- a/Doc/library/tkinter.rst
+++ b/Doc/library/tkinter.rst
@@ -818,6 +818,10 @@
 deleted, the image data is deleted as well, and Tk will display an empty box
 wherever the image was used.
 
+.. seealso::
+
+    The `Pillow <http://python-pillow.org/>`_ package adds support for
+    formats such as BMP, JPEG, TIFF, and WebP, among others.
 
 .. _tkinter-file-handlers: