Django form with choices but also with freetext option?

I would recommend a custom Widget approach, HTML5 allows you to have a free text input with a dropdown list which would work as a pick-one-or-write-other type of field, this is how I made it:

fields.py

from django import forms

class ListTextWidget(forms.TextInput):
    def __init__(self, data_list, name, *args, **kwargs):
        super(ListTextWidget, self).__init__(*args, **kwargs)
        self._name = name
        self._list = data_list
        self.attrs.update({'list':'list__%s' % self._name})

    def render(self, name, value, attrs=None, renderer=None):
        text_html = super(ListTextWidget, self).render(name, value, attrs=attrs)
        data_list="<datalist id="list__%s">" % self._name
        for item in self._list:
            data_list += '<option value="%s">' % item
        data_list += '</datalist>'

        return (text_html + data_list)

forms.py

from django import forms
from myapp.fields import ListTextWidget

class FormForm(forms.Form):
   char_field_with_list = forms.CharField(required=True)

   def __init__(self, *args, **kwargs):
      _country_list = kwargs.pop('data_list', None)
      super(FormForm, self).__init__(*args, **kwargs)

    # the "name" parameter will allow you to use the same widget more than once in the same
    # form, not setting this parameter differently will cuse all inputs display the
    # same list.
       self.fields['char_field_with_list'].widget = ListTextWidget(data_list=_country_list, name="country-list")

views.py

from myapp.forms import FormForm

def country_form(request):
    # instead of hardcoding a list you could make a query of a model, as long as
    # it has a __str__() method you should be able to display it.
    country_list = ('Mexico', 'USA', 'China', 'France')
    form = FormForm(data_list=country_list)

    return render(request, 'my_app/country-form.html', {
        'form': form
    })