Python changelog fragments

Create and validate changelog fragments for Python projects that use antsibull-changelog (Ansible collections), towncrier, or scriv (most other Python packages). All three solve the same problem, a PR-scoped snippet that gets collected into the real changelog at release time, but they use different file formats, config sections, and directory conventions.

1. Identify the tool in use

Check for these markers before doing anything else, they determine which reference file and commands apply:

towncrier and scriv both commonly use a directory literally named changelog.d/, so disambiguate by the config section name, not the directory. If neither marker exists, ask which tool the project uses rather than guessing, the fragment format and directory are not interchangeable.

2. Load the tool-specific reference