Bring back nest-asyncio to allow running notebooks with latest release (

#4472)

Fixes #4470
Review: @viathor

docs/operators_and_observables.ipynb

@@ -800,6 +800,21 @@
     "The pattern for computing $\\text{Tr} [ \\rho O ]$ is shown below."
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "id": "7d55b4675db8"
+   },
+   "outputs": [],
+   "source": [
+    "# Install nest-asyncio to run collector inside jupyter.\n",
+    "# This will no longer be necessary in cirq 0.13+.\n",
+    "!pip install nest-asyncio\n",
+    "import nest_asyncio\n",
+    "nest_asyncio.apply()   "
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,