Closes Issue 21262: New method assert_not_called for Mock.

It raises AssertionError if the mock has been called.
diff --git a/NEWS b/NEWS
index ab79a62..93db80f 100644
--- a/NEWS
+++ b/NEWS
@@ -1,3 +1,6 @@
+- Issue #21262: New method assert_not_called for Mock.
+  It raises AssertionError if the mock has been called.
+
 - Issue #21238: New keyword argument `unsafe` to Mock. It raises
   `AttributeError` incase of an attribute startswith assert or assret.