Fix #1537721: add writeheader() method to csv.DictWriter.

Reviewed by skip.montanaro and thomas.wouters.
diff --git a/Misc/NEWS b/Misc/NEWS
index ff04b28..8b91de8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -53,6 +53,8 @@
 
 - Issue #5801: removed spurious empty lines in wsgiref.
 
+- Issue #1537721: Add a writeheader() method to csv.DictWriter.
+
 Extension Modules
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