bump_country_package.py broken after pyproject.toml migration

[anth-volk]

Problem

The migration to pyproject.toml in #3277 deleted setup.py and replaced changelog_entry.yaml with changelog.d/ towncrier fragments. However, gcp/bump_country_package.py — the script called by policyengine-us and policyengine-uk CI to automatically open version-bump PRs — was not updated. It still:

1. Reads/writes setup.py (line 29-43), which no longer exists. This causes a FileNotFoundError at runtime.
2. Writes changelog_entry.yaml (lines 53-56), which is the deprecated changelog format. The repo now uses changelog.d/ fragments.
3. Has an indentation bug on line 40 where setup_py.replace(...) is outside the if match: block, which would cause an AttributeError if the regex didn't match.

As a result, no automated country-package bump PRs can be created until this is fixed.

Expected fix

• Read/write pyproject.toml instead of setup.py
• Create a changelog.d/{branch_name}.changed.md fragment instead of changelog_entry.yaml
• Fix the indentation bug so the replace is inside the if match: block