FIX: smart_text is deprecated; use smart_str. (Django deprecation)

[FlipperPA]

Fixes https://github.com/prymitive/bootstrap-breadcrumbs/issues/64

My test suite has started throwing deprecation warnings about this being fully removed in Django 4.0.

smart_str() can be viewed here: https://github.com/django/django/blob/master/django/utils/encoding.py#L19