title: Django multiple select form - TypeError: int() argument must be a string or a number, not 'list'
body: <p>I have a form like so:</p>
  
  <pre><code>class PartnerProductsForm(forms.Form):
      product = forms.ModelChoiceField(
          queryset=Product.objects.all(),
          widget=forms.CheckboxSelectMultiple(
              attrs={"checked": ""}
          ),
          empty_label=None,
      )
  </code></pre>
  
  <p>And my views:</p>
  
  <pre><code>...
  product_form = PartnerProductsForm(request.POST or None)
      if product_form.is_valid():
          # do stuff
  </code></pre>
  
  <p>But when I submit I get the following error:</p>
  
  <pre><code>TypeError: int() argument must be a string or a number, not 'list'
  </code></pre>
  
  <p>It seems like the form validation is expecting an int, but of course I will be retuning a list of the checked options. How am I supposed to deal with this?</p>