maintenance.sh: Silence pip install -r requirements.txt (otherwise di…

…splayed in every cron job mail) (#2545)

maintenance.sh

@@ -6,7 +6,7 @@ cd $workdir
 function update_venv()
 {
     if [ -d "venv" ]; then
-        venv/bin/pip install -r requirements.txt
+        venv/bin/pip install -r requirements.txt >/dev/null
     fi
 }