#27364: Deprecate invalid escape strings in str/byutes.

Patch by Emanuel Barry, reviewed by Serhiy Storchaka and Martin Panter.
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 9ab624e..2684b94 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -10,6 +10,7 @@
 import itertools
 import operator
 import struct
+import string
 import sys
 import unittest
 import warnings
@@ -2752,6 +2753,12 @@
         support.check_free_after_iterating(self, iter, str)
         support.check_free_after_iterating(self, reversed, str)
 
+    def test_invalid_sequences(self):
+        for letter in string.ascii_letters + "89": # 0-7 are octal escapes
+            if letter in "abfnrtuvxNU":
+                continue
+            with self.assertWarns(DeprecationWarning):
+                eval(r"'\%s'" % letter)
 
 class StringModuleTest(unittest.TestCase):
     def test_formatter_parser(self):