bpo-6135: Fix subprocess.check_output doc to mention changes in 3.6 (GH-5564) (GH-5573)

Fixes the documentation for `subprocess.check_output()` not mentioning that the encoding and errors parameters were added in 3.6.
(cherry picked from commit fc1ce810)
Co-authored-by: Brice Gros <brice-gros@users.noreply.github.com>

Doc/library/subprocess.rst

@@ -956,6 +956,9 @@ calls these functions.
    .. versionchanged:: 3.4
       Support for the *input* keyword argument was added.
 
+   .. versionchanged:: 3.6
+      *encoding* and *errors* were added.  See :func:`run` for details.
+
 .. _subprocess-replacements:
 
 Replacing Older Functions with the :mod:`subprocess` Module