title: How to prevent user to be both of types?
body: <p>I have a model like from <a href="https://stackoverflow.com/a/25842236/11486181">this answer</a>:</p>
  
  <pre class="lang-py prettyprint-override"><code>class MyUser(AbstractUser):
      is_student = models.BooleanField(default=False)
      is_teacher = models.BooleanField(default=False)
  </code></pre>
  
  <p>Is it possible to prevent create new user both of types simultaneously at model level? In other words, it could be ONLY <code>is_student == False</code> and <code>is_teacher == True</code> or vise versa.</p>