Update docstring of tempfile._RandomNameSequence (GH-6414)


When bpo-12015 got resolved by increasing the length of the
random string generated by _RandomNameSequence from six to eight
characters, the docstring of the class was not adjusted accordingly.
(cherry picked from commit 9c463ec88ba21764f6fff8e01d6045a932a89438)

Co-authored-by: Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de>
diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index 71ecafa..e5b7a92 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -132,7 +132,7 @@
 class _RandomNameSequence:
     """An instance of _RandomNameSequence generates an endless
     sequence of unpredictable strings which can safely be incorporated
-    into file names.  Each string is six characters long.  Multiple
+    into file names.  Each string is eight characters long.  Multiple
     threads can safely use the same instance at the same time.
 
     _RandomNameSequence is an iterator."""