backport hmac.compare_digest to partially implement PEP 466 (closes #21306)

Backport from Alex Gaynor.
diff --git a/Misc/NEWS b/Misc/NEWS
index 57d1f24..6548956 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -49,6 +49,9 @@
 Library
 -------
 
+- Issue #21306: Backport hmac.compare_digest from Python 3. This is part of PEP
+  466.
+
 - Issue #21321: itertools.islice() now releases the reference to the source
   iterator when the slice is exhausted.  Patch by Anton Afanasyev.