Make Lorenz differential equations example compatible with Python 3

Thanks to @franktoffel for pointing this out in  #7350

examples/Interactive Widgets/Lorenz Differential Equations.ipynb

@@ -111,8 +111,9 b''
     "    ax.set_ylim((-35, 35))\n",
     "    ax.set_zlim((5, 55))\n",
     "    \n",
-    "    def lorenz_deriv((x, y, z), t0, sigma=sigma, beta=beta, rho=rho):\n",
+    "    def lorenz_deriv(x_y_z, t0, sigma=sigma, beta=beta, rho=rho):\n",
     "        \"\"\"Compute the time-derivative of a Lorentz system.\"\"\"\n",
+    "        x, y, z = x_y_z\n",
     "        return [sigma * (y - x), x * (rho - z) - y, x * y - beta * z]\n",
     "\n",
     "    # Choose random starting points, uniformly distributed from -15 to 15\n",
@@ -13428,7 +13429,12 b''
    ]
   }
  ],
- "metadata": {},
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "name": "python3"
+  }
+ },
  "nbformat": 4,
  "nbformat_minor": 0
-}

  No newline at end of file
+}