change the named tuple returned by inspect.getfullargspec to have a 'kwonlydefaults' (as claimed by the docs) attribute instead of 'kwdefaults'

Fixes #4307
Reviewed by Christian
diff --git a/Misc/NEWS b/Misc/NEWS
index 24afb0e..ecc808f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -19,6 +19,9 @@
 Library
 -------
 
+- Issue #4307: The named tuple that ``inspect.getfullargspec()`` returns now
+  uses ``kwonlydefaults`` instead of ``kwdefaults``.
+
 - Issue #4298: Fix a segfault when pickle.loads is passed a ill-formed input.
 
 - Issue #4283: Fix a left-over "iteritems" call in distutils.