pylightning: Fix the parameter `label` of `sendpay`

`description` is renamed as `label` in 0.7.0

contrib/pylightning/lightning/lightning.py

@@ -804,18 +804,37 @@ class LightningRpc(UnixDomainSocketRpc):
         }
         return self.call("plugin", payload)
 
-    def sendpay(self, route, payment_hash, description=None, msatoshi=None):
+    def _deprecated_sendpay(self, route, payment_hash, description, msatoshi=None):
+        warnings.warn("sendpay: the 'description' field is renamed 'label' : expect removal"
+                      " in early-2020",
+                      DeprecationWarning)
+        payload = {
+            "route": route,
+            "payment_hash": payment_hash,
+            "label": description,
+            "msatoshi": msatoshi,
+        }
+        return self.call("sendpay", payload)
+
+    def sendpay(self, route, payment_hash, *args, **kwargs):
         """
         Send along {route} in return for preimage of {payment_hash}
         """
+
+        if 'description' in kwargs:
+            return self._deprecated_sendpay(route, payment_hash, *args, **kwargs)
+
+        def _sendpay(route, payment_hash, label=None, msatoshi=None):
             payload = {
                 "route": route,
                 "payment_hash": payment_hash,
-            "description": description,
+                "label": label,
                 "msatoshi": msatoshi,
             }
             return self.call("sendpay", payload)
 
+        return _sendpay(route, payment_hash, *args, **kwargs)
+
     def setchannelfee(self, id, base=None, ppm=None):
         """
         Set routing fees for a channel/peer {id} (or 'all'). {base} is a value in millisatoshi