fix ci (#4230)

Author: MarcoGorelli

docs/source/notebooks/AR.ipynb

@@ -49,7 +49,7 @@
     "%config InlineBackend.figure_format = 'retina'\n",
     "RANDOM_SEED = 8927\n",
     "np.random.seed(RANDOM_SEED)\n",
-    "az.style.use('arviz-darkgrid')"
+    "az.style.use(\"arviz-darkgrid\")"
    ]
   },
   {
@@ -281,7 +281,7 @@
     "    idata,\n",
     "    lines=[\n",
     "        (\"theta\", {}, true_theta),\n",
-    "        (\"tau\", {}, true_sigma**-2),\n",
+    "        (\"tau\", {}, true_sigma ** -2),\n",
     "        (\"center\", {}, true_center),\n",
     "    ],\n",
     ");"