Roll back API to 0.13.13 and pin core/urllib3

[anth-volk]

Fixes #1475

Summary

• restore tracked files to the exact 0.13.13 baseline from 0f9fe32b0dce05f7eba0f457b194620a29c5d97b
• pin policyengine_core<=3.23.6
• pin urllib3<=1.26.20
• use the older changelog flow by carrying a valid changelog_entry.yaml fragment on the branch, while leaving versioned files at 0.13.13

Tree Equivalence

• the first rollback commit in this PR is 6fb1db067c846e34a8ec7cec0f4d92eaa5d369d6
• it is not the same commit object as 0f9fe32b0dce05f7eba0f457b194620a29c5d97b, because it has different commit metadata and parentage
• its tracked-file tree is identical to 0f9fe32b0dce05f7eba0f457b194620a29c5d97b
• 6fb1db0^{tree} = 7fdf663c197b0994589de3affa9e2f6da0311f6c
• 0f9fe32^{tree} = 7fdf663c197b0994589de3affa9e2f6da0311f6c
• git diff 0f9fe32 6fb1db0 is empty

Changelog Handling

• this rollback branch intentionally does not pre-generate a 0.13.14 version bump
• policyengine_household_api/constants.py remains at 0.13.13
• the branch includes a valid old-style changelog_entry.yaml element instead
• note: we may need to recreate the changelog generated by the new changelog style in a future PR.

Validation

• ran git ls-files '*.py' | xargs black -l 79
• ran git ls-files '*.py' | xargs black -l 79 --check