bpo-29728: Provide socket.TCP_NOTSENT_LOWAT (#477)

* Provide socket.TCP_NOTSENT_LOWAT
* New TCP option available on recent MacOS and Linux.
* Document addition of TCP_NOTSENT_LOWAT
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index 63e87e6..6d5c256 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -7507,6 +7507,9 @@
 #ifdef  TCP_USER_TIMEOUT
     PyModule_AddIntMacro(m, TCP_USER_TIMEOUT);
 #endif
+#ifdef  TCP_NOTSENT_LOWAT
+    PyModule_AddIntMacro(m, TCP_NOTSENT_LOWAT);
+#endif
 
     /* IPX options */
 #ifdef  IPX_TYPE