Need a way to set the order of fragments

[hynek]

The new way of defining custom fragments uses a dictionary that is unfortunately sorted alphabetically by tomli.

For example this is from my Markdown tutorial in #436:

{'tool': {'towncrier': {'directory': 'changelog.d',
                        'filename': 'CHANGELOG.md',
                        'fragment': {'added': {'name': 'Added'},
                                     'changed': {'name': 'Changed'},
                                     'deprecated': {'name': 'Deprecated'},
                                     'fixed': {'name': 'Fixed'},
                                     'removed': {'name': 'Removed'},
                                     'security': {'name': 'Security'}},
                        'issue_format': '[#{issue}](https://github.com/twisted/my-project/issues/{issue})',
                        'start_string': '<!-- towncrier release notes start '
                                        '-->\n',
                        'template': 'changelog.d/changelog_template.jinja',
                        'title_format': '## '
                                        '[{version}](https://github.com/twisted/my-project/tree/{version}) '
                                        '- {project_date}',
                        'underlines': ['', '', '']}}}

The fragment types are defined as:

[tool.towncrier.fragment.security]
name = "Security"

[tool.towncrier.fragment.removed]
name = "Removed"

[tool.towncrier.fragment.deprecated]
name = "Deprecated"

[tool.towncrier.fragment.added]
name = "Added"

[tool.towncrier.fragment.changed]
name = "Changed"

[tool.towncrier.fragment.fixed]
name = "Fixed"

In hindsight, it's unfortunate we've switched away from using a list for that but it is what it is.

I can think of two way to solve this:

1. add an order key to the fragments:

[tool.towncrier.fragment.security]
name = "Security"
order = 1

[tool.towncrier.fragment.removed]
name = "Removed"
order = 2

2. add an option:

[tool.towncrier]
category_order = ["security", "removed", "..."]

I think I prefer option 2, because you don't have to re-number all entries if you change the order.

The question is what would happen if a catefory is missing – I would suggest an error.

[hynek]

Looks like this is a fairly new change in #370 – I wonder if we should just un-deprecate it.