Add towncrier for CHANGELOG management

Fix #154

changelog/154.doc

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+Introduced ``towncrier`` for ``CHANGELOG`` management.

changelog/_template.rst

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+{% for section in sections %}
+{% set underline = "-" %}
+{% if section %}
+{{section}}
+{{ underline * section|length }}{% set underline = "~" %}
+
+{% endif %}
+{% if sections[section] %}
+{% for category, val in definitions.items() if category in sections[section] and category != 'trivial' %}
+
+{{ definitions[category]['name'] }}
+{{ underline * definitions[category]['name']|length }}
+
+{% if definitions[category]['showcontent'] %}
+{% for text, values in sections[section][category]|dictsort(by='value') %}
+- {{ text }}{% if category != 'vendor' %} ({{ values|sort|join(', ') }}){% endif %}
+
+
+{% endfor %}
+{% else %}
+- {{ sections[section][category]['']|sort|join(', ') }}
+
+
+{% endif %}
+{% if sections[section][category]|length == 0 %}
+
+No significant changes.
+
+
+{% else %}
+{% endif %}
+{% endfor %}
+{% else %}
+
+No significant changes.
+
+
+{% endif %}
+{% endfor %}