fix: redirect server subprocess output to /dev/null

Author: kibitzing

examples/OpenEnv_Tutorial.ipynb

@@ -817,8 +817,8 @@
     "         \"OPENSPIEL_GAME\": \"catch\",\n",
     "         \"OPENSPIEL_AGENT_PLAYER\": \"0\",\n",
     "         \"OPENSPIEL_OPPONENT_POLICY\": \"random\"},\n",
-    "    stdout=subprocess.PIPE,\n",
-    "    stderr=subprocess.PIPE,\n",
+    "    stdout=subprocess.DEVNULL,\n",
+    "    stderr=subprocess.DEVNULL,\n",
     "    text=True,\n",
     "    cwd=work_dir\n",
     ")\n",