Server: add missing 'restart_hook' missing

Change-Id: Iac6349c86ee7cc5c55177e881a51a240df260d6f
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/31004
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>
Reviewed-by: Jens Krueger <jens.krueger@frm2.tum.de>

frappy/server.py

@@ -125,6 +125,9 @@ class Server:
     def unknown_options(self, cls, options):
         return f"{cls.__name__} class don't know how to handle option(s): {', '.join(options)}"
 
+    def restart_hook(self):
+        pass
+
     def run(self):
         while self._restart:
             self._restart = False