backport hashlib.pbkdf2_hmac per PEP 466 (closes #21304)

Backport by Alex Gaynor.
diff --git a/Misc/NEWS b/Misc/NEWS
index 012acd7..b4fe8fb 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -18,6 +18,9 @@
 Library
 -------
 
+- Issue #21304: Backport the key derivation function hashlib.pbkdf2_hmac from
+  Python 3 per PEP 466.
+
 - Issue #21552: Fixed possible integer overflow of too long string lengths in
   the tkinter module on 64-bit platforms.