bpo-34839: Add a 'before 3.6' in the section 'warnings' of doctest (pythonGH-9736)

Author: matrixise

Doc/library/doctest.rst

@@ -759,23 +759,27 @@ Warnings
 :mod:`doctest` is serious about requiring exact matches in expected output. If
 even a single character doesn't match, the test fails. This will probably
 surprise you a few times, as you learn exactly what Python does and doesn't
-guarantee about output. For example, when printing a dict, Python doesn't
-guarantee that the key-value pairs will be printed in any particular order, so a
-test like ::
+guarantee about output. For example, when printing a set, Python doesn't
+guarantee that the element is printed in any particular order, so a test like ::
 
  >>> foo()
- {"Hermione": "hippogryph", "Harry": "broomstick"}
+ {"Hermione", "Harry"}
 
 is vulnerable! One workaround is to do ::
 
- >>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
+ >>> foo() == {"Hermione", "Harry"}
  True
 
 instead. Another is to do ::
 
- >>> d = sorted(foo().items())
+ >>> d = sorted(foo())
  >>> d
- [('Harry', 'broomstick'), ('Hermione', 'hippogryph')]
+ ['Harry', 'Hermione']
+
+.. note::
+
+ Before Python 3.6, when printing a dict, Python did not guarantee that
+ the key-value pairs was printed in any particular order.
 
 There are others, but you get the idea.