synchroniser : use with when using Popen

Creating a subprocess with Popen is allocating ressources (according to
pylint) so wrap it in a with to avoid problems.

Also link to unison documentation for the return code value.

src/unisync/synchroniser.py

@@ -123,7 +123,7 @@ class Synchroniser:
                 f"{self.remote_user}@{self.remote_ip}",
                 "-p", str(self.remote_port)
                 ]
-        master_ssh = subprocess.Popen(command)
+        with subprocess.Popen(command) as master_ssh:
             # TODO: Raise an exception instead of changing the return value
             try:
                 ret_code = master_ssh.wait(timeout=connection_timeout)
@@ -151,8 +151,10 @@ class Synchroniser:
                 f"{self.remote_user}@{self.remote_ip}",
                 "-p", str(self.remote_port)
                 ]
-        close = subprocess.Popen(command)
-        return close.wait()
+        with subprocess.Popen(command) as close:
+            retval = close.wait()
+
+        return retval
 
     def sync_files(self, paths:list, force:bool=False) -> None:
         """Synchronises the files.
@@ -254,8 +256,10 @@ class Synchroniser:
         for arg in other:
             command.append(arg)
 
-        proc = subprocess.Popen(command)
+        with subprocess.Popen(command) as proc:
             ret_code = proc.wait()
+
+        # See unison manual section 6.11
         if ret_code == 3:
             raise FatalSyncError("Synchronisation could not be completed")