Fix duplicate top line in NEWS.rst

It looks like towncrier unconditionally writes top_line
when it is defined in the title_format configuration.
So we must not repeat it in the template.
When running towncrier with --draft, it works differently
so in that case the top line is not shown...

tools/news/template.rst

@@ -1,6 +1,6 @@
 {% set underline = "=" %}
-{{ top_line }}
-{{ underline * top_line|length }}
+
+{{ underline * ((top_line)|length) }}
 {% for section in sections %}
 {% set underline = "-" %}
 {% if section %}