Description

towncrier create should work better with sections if defined in the configuration file.

This adds a new towncrier create --section <section name> option, or if you're using the new interactive create then you'll be asked for the section if there are multiple ones defined in your config:

$ towncrier create --content "Sections!"
Pick a section
 1. Backend
 2. Frontend
Section (1, 2) [1]: 2
Issue number (`+` for none): 603
Fragment type (feature, bugfix, doc, removal, misc): feature
Created news fragment at /home/chris/demo/changes/frontend/603.feature.rst

If you use sections and none have an empty path, you must now specify the section when creating a new news fragment (if one does have an empty path, that section will be used by default).

This PR also deduplicates the code that determines the base fragment path which was done in three places!

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure local test run is green.
• Create a file in src/towncrier/newsfragments/. Describe your
  change and include important information. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• Ensure docs/tutorial.rst is still up-to-date.
• If you add new CLI arguments (or change the meaning of existing ones), make sure docs/cli.rst reflects those changes.
• If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.