bpo-39068: Fix race condition in base64 (GH-17627)

There was a race condition in base64 in lazy initialization of multiple globals.
diff --git a/Misc/ACKS b/Misc/ACKS
index 80e51f9..211455b 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1659,6 +1659,7 @@
 Frank Stajano
 Joel Stanley
 Kyle Stanley
+Brandon Stansbury
 Anthony Starks
 David Steele
 Oliver Steele