Making a text bold shouldn't be done in the view. Indeed, the view should not take care of formatting (which is the role of templates). What you can do however, is adding an extra variable to the rendering context, and depending on its value, make the text bold or not in the template.
For example :
In the view :
#...
is_important = True if something else False
extra_context.update({'is_important': is_important})
#...
In the template :
...
{% if is_important %}<bold>{{ text_to_render }}</bold>{% else %}{{ text_to_render }}{% endif %}
...
But more generally, deciding if a text is bold or not, is not even formatting, rather styling and should not be made in the template (so you shouldn't use the <bold> markup). So I would suggest :
...
<span {% if is_important %}class="is-important"{% endif %}>
{{ text_to_render }}
</span>
...
And a stylesheet :
.is-important{
font-weight: bold;
}