bpo-33297: Mention Pillow to work with more image formats. (#6505)

Also update PIL doc references to Pillow.
diff --git a/Doc/distutils/introduction.rst b/Doc/distutils/introduction.rst
index 8f46bd7..a385559 100644
--- a/Doc/distutils/introduction.rst
+++ b/Doc/distutils/introduction.rst
@@ -193,8 +193,8 @@
 module distribution
    a collection of Python modules distributed together as a single downloadable
    resource and meant to be installed *en masse*.  Examples of some well-known
-   module distributions are NumPy, SciPy, PIL (the Python Imaging
-   Library), or mxBase.  (This would be called a *package*, except that term is
+   module distributions are NumPy, SciPy, Pillow,
+   or mxBase.  (This would be called a *package*, except that term is
    already taken in the Python context: a single module distribution may contain
    zero, one, or many Python packages.)
 
diff --git a/Doc/library/tkinter.rst b/Doc/library/tkinter.rst
index b99dc8e..a6a2e47 100644
--- a/Doc/library/tkinter.rst
+++ b/Doc/library/tkinter.rst
@@ -800,6 +800,10 @@
 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:
 
diff --git a/Doc/tutorial/modules.rst b/Doc/tutorial/modules.rst
index 584d4fd..3f68932 100644
--- a/Doc/tutorial/modules.rst
+++ b/Doc/tutorial/modules.rst
@@ -382,7 +382,7 @@
 named ``B`` in a package named ``A``.  Just like the use of modules saves the
 authors of different modules from having to worry about each other's global
 variable names, the use of dotted module names saves the authors of multi-module
-packages like NumPy or the Python Imaging Library from having to worry about
+packages like NumPy or Pillow from having to worry about
 each other's module names.
 
 Suppose you want to design a collection of modules (a "package") for the uniform