Add a space after header, so reST rendering isn't mangled. (#1429)

* Add a space after header, so reST rendering isn't mangled.

* Add newline in template, too.

CHANGES.rst

 v40.0.0
 -------
+
 * #1342: Drop support for Python 3.3.
 * #1366: In package_index, fixed handling of encoded entities in URLs.
 * #1383: In pkg_resources VendorImporter, avoid removing packages imported from the root.

towncrier_template.rst

@@ -2,6 +2,7 @@
 {% set underline = underlines[0] %}{% if section %}{{section}}
 {{ underline * section|length }}
 {% endif %}
+
 {% if sections[section] %}
 {% for category, val in definitions.items() if category in sections[section]%}
 {% if definitions[category]['showcontent'] %}