bpo-39426: Fix outdated default and highest protocols in docs (GH-18154)


Some portions of the pickle documentation hadn't been updated for the pickle protocol changes in Python 3.8 (new protocol 5, default protocol 4). This PR fixes those docs.

https://bugs.python.org/issue39426
(cherry picked from commit e9652e8d58392f5022759ba06b444ce970eb12db)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
diff --git a/Lib/pickle.py b/Lib/pickle.py
index 71aa57d..515cb8a 100644
--- a/Lib/pickle.py
+++ b/Lib/pickle.py
@@ -409,9 +409,9 @@
         """This takes a binary file for writing a pickle data stream.
 
         The optional *protocol* argument tells the pickler to use the
-        given protocol; supported protocols are 0, 1, 2, 3 and 4.  The
-        default protocol is 4. It was introduced in Python 3.4, it is
-        incompatible with previous versions.
+        given protocol; supported protocols are 0, 1, 2, 3, 4 and 5.
+        The default protocol is 4. It was introduced in Python 3.4, and
+        is incompatible with previous versions.
 
         Specifying a negative protocol version selects the highest
         protocol version supported.  The higher the protocol used, the