Widget with plotly figure stalls otter assign

[bdsomer]

Describe the bug
Any cell in a notebook that has a plotly.graph_objs.Figure in an ipywidgets.interacting causes otter assign to never finish running. For example, this cell stalls otter assign:

import ipywidgets
import plotly

slider = ipywidgets.FloatSlider()
def on_slider_change(val):
    fig = plotly.graph_objs.Figure()
    plotly.offline.iplot(fig)

display(ipywidgets.interactive(on_slider_change, val = slider))

To Reproduce
Steps to reproduce the behavior:
Run otter assign on the following notebook:

{
 "cells": [
  {
   "cell_type": "raw",
   "metadata": {},
   "source": [
    "# BEGIN QUESTION\n",
    "name: q1"
   ]
  },
  {
   "cell_type": "raw",
   "metadata": {},
   "source": [
    "# BEGIN SOLUTION"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "q1 = 1 # SOLUTION"
   ]
  },
  {
   "cell_type": "raw",
   "metadata": {},
   "source": [
    "# END SOLUTION"
   ]
  },
  {
   "cell_type": "raw",
   "metadata": {},
   "source": [
    "# BEGIN TESTS"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "assert q1 == 1"
   ]
  },
  {
   "cell_type": "raw",
   "metadata": {},
   "source": [
    "# END TESTS"
   ]
  },
  {
   "cell_type": "raw",
   "metadata": {},
   "source": [
    "# END QUESTION"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import ipywidgets\n",
    "import plotly\n",
    "\n",
    "slider = ipywidgets.FloatSlider()\n",
    "def on_slider_change(val):\n",
    "    fig = plotly.graph_objs.Figure()\n",
    "    plotly.offline.iplot(fig)\n",
    "\n",
    "display(ipywidgets.interactive(on_slider_change, val = slider))"
   ]
  }
 ],
 "metadata": {
  "@webio": {
   "lastCommId": null,
   "lastKernelId": null
  },
  "kernelspec": {
   "display_name": "Python 3 (ipykernel)",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.12.2"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 4
}

Expected behavior
otter assign completes normally (like it does without the problematic cell in the example)

Versions
Python version: 3.12.2
Otter-Grader version: 5.6.0

Additional context
Otter version 5.0.0 is the first version in which this bug exists. The line of code on which otter assign stalls appears to be

otter-grader/otter/execute/__init__.py

Line 100 in d897d6a

executed_nb, _ = ep.preprocess(nb)

Based on this, this issue may actually be an issue with nbconvert.

[chrispyles]

This sounds similar to #755. Can you try that fix and see if it works?