synchroniser : update_links : use run instead of Popen

using Popen causes the command to return before making the connection so
the closing of the master connection (in runners) happens before we had
time to log into the server forcing us to complete an extra login step.

src/unisync/synchroniser.py

@@ -270,7 +270,7 @@ class Synchroniser:
                 link_background_wrapper
                 ]
 
-        link_update_process = subprocess.Popen(command, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        link_update_process = subprocess.run(command, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
 
         if not background:
             print("Starting links update.")