bpo-39377: json: Update doc about the encoding option.

Add What's New entry.
python · methane · methane · methane · Jan 20, 2020 · the-knights-who-say-ni
diff --git a/Doc/library/json.rst b/Doc/library/json.rst
   .. versionchanged:: 3.6
      *s* can now be of type :class:`bytes` or :class:`bytearray`. The
      input encoding should be UTF-8, UTF-16 or UTF-32.

+   .. versionchanged:: 3.9
+      *encoding* keyword argument is removed.
-      *encoding* keyword argument is removed.
+      The keyword argument *encoding* has been removed.
diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
+  Python 3.1, it was deprecated and ignored.  And using it was emitting
+  :exc:`DeprecationWarning` since Python 3.8.