Use raw strings in the re module examples. (pythonGH-4616)

(cherry picked from commit c615be5)

Doc/library/re.rst

@@ -668,11 +668,11 @@ form.
    splits occur, and the remainder of the string is returned as the final element
    of the list. ::
 
-      >>> re.split('\W+', 'Words, words, words.')
+      >>> re.split(r'\W+', 'Words, words, words.')
       ['Words', 'words', 'words', '']
-      >>> re.split('(\W+)', 'Words, words, words.')
+      >>> re.split(r'(\W+)', 'Words, words, words.')
       ['Words', ', ', 'words', ', ', 'words', '.', '']
-      >>> re.split('\W+', 'Words, words, words.', 1)
+      >>> re.split(r'\W+', 'Words, words, words.', 1)
       ['Words', 'words, words.']
       >>> re.split('[a-f]+', '0a3B9', flags=re.IGNORECASE)
       ['0', '3', '9']
@@ -681,7 +681,7 @@ form.
    the string, the result will start with an empty string.  The same holds for
    the end of the string::
 
-      >>> re.split('(\W+)', '...words, words...')
+      >>> re.split(r'(\W+)', '...words, words...')
       ['', '...', 'words', ', ', 'words', '...', '']
 
    That way, separator components are always found at the same relative