Set absolute URLs prior to uploading to PyPI (#5038)

## Summary

Closes https://github.com/astral-sh/uv/issues/5030.

.github/workflows/ci.yml

@@ -45,6 +45,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - uses: actions/setup-python@v5
+        with:
+          python-version: 3.12
+
       - name: "Install Rustfmt"
         run: rustup component add rustfmt
 
@@ -60,6 +64,9 @@ jobs:
       - name: "Ruff check"
         run: pipx run ruff check .
 
+      - name: "README check"
+        run: python scripts/transform_readme.py --target pypi
+
   cargo-clippy:
     needs: determine_changes
     if: ${{ github.repository == 'astral-sh/uv' && (needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main') }}