Попробуйте использовать class = "pull-left" вместо class = "float-left"
<div class="container-fluid">
<div class="row d-flex justify-content-center">
<div class="col-lg-6 col-md-6 col-xs-12 col-sm-12">
<div class="list-group shadow">
{% for newsletter in newsletter_list %}
<a href="{{newsletter.slug}}" class="list-group-item list-group-item-action">
<span class="pull-left">?</span>
<span class="pull-right">Drago</span>
</a>
{% endfor %}
</div>
</div>
</div>