Fixes release blocker issue #3492 and #3790.

Make zlib and zipimport to return bytes instead of bytearray and use bytes
rather than bytearray for their internal leftover data storages.
diff --git a/Misc/NEWS b/Misc/NEWS
index bc6ede7..cbbe447 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -137,6 +137,10 @@
 
 - The _bytesio and _stringio modules are now compiled into the python binary.
 
+- Issue #3492 and #3790: Fixed the zlib module and zipimport module uses of
+  mutable bytearray objects where they should have been using immutable bytes.
+
+
 Tools/Demos
 -----------