Use `namedtuple` in result of `{% regroup %}`

[Baptiste Mispelon]

Currently, {% regroup %} returns a list of dictionaries with two key (grouper and list).

I propose chaning that to return a namedtuple with the same fields.

This should be backwards-compatible because attribute access and key access inside template uses the same syntax.

The added benefit would be that you could now write:

{% regroup places by country as country_list %}

<ul>
{% for country, cities in country_list %}
    <li>{{ country }}
    <ul>
        {% for city in cities %}
          <li>{{ city.name }}: {{ city.population }}</li>
        {% endfor %}
    </ul>
    </li>
{% endfor %}
</ul>