Render top_line explicitly in towncrier template

This allows sphinxcontrib-towncrier to render it in the documentation.
The rendering of NEWS.rst is unchanged since Towncrier detects
automatically if that line exists and does not render a duplicated
section title.

tools/news/template.rst

-{% for section in sections %}
+{{ top_line -}}
+
+{%- for section in sections %}
 {% set underline = "-" %}
 {% if section %}
-{{section}}
+{{ section }}
 {{ underline * section|length }}{% set underline = "~" %}
 
 {% endif %}