go/neo/client_test: NEO/py now only supports M encoding

With the switch to the msgpack encoding, the NEO/py server
now only supports the M encoding.

go/neo/client_test.go

@@ -90,7 +90,7 @@ func (_ *NEOPySrv) Bugs() []string {
 }
 
 func (_ *NEOPySrv) BugEncFixed() (bool, proto.Encoding) {
-	return true, 'N' // NEO/py 1.12
+	return true, 'M' // NEO/py > 1.12
 }