Versioning workflow fails on main: towncrier not installed

[anth-volk]

Bug

The "Versioning updates" CI workflow fails on the main branch at the "Build changelog" step.

Failed run: https://github.com/PolicyEngine/policyengine.py/actions/runs/22362677665/job/64720229780

Root cause

The main branch's Makefile changelog target uses towncrier, while the 0.x branch's uses yaml-changelog. The versioning workflow triggers on both branches but only installed yaml-changelog:

- name: Build changelog
  run: pip install yaml-changelog && make changelog

When the workflow runs on main, towncrier is not found (exit code 127).

Fix

Install both packages so the workflow works for both branches:

run: pip install yaml-changelog towncrier && make changelog

Each branch's Makefile will call whichever tool it needs.