Eric Snow's implementation of PEP 421.

Issue 14673: Add sys.implementation
diff --git a/Include/Python.h b/Include/Python.h
index 6affbf5..a78a721 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -101,6 +101,7 @@
 #include "warnings.h"
 #include "weakrefobject.h"
 #include "structseq.h"
+#include "namespaceobject.h"
 
 #include "codecs.h"
 #include "pyerrors.h"