fix: bump pdm-project/setup-pdm from 3 to 4 (#15)

Bumps [pdm-project/setup-pdm](https://github.com/pdm-project/setup-pdm) from 3 to 4.
- [Release notes](https://github.com/pdm-project/setup-pdm/releases)
- [Commits](https://github.com/pdm-project/setup-pdm/compare/v3...v4)

---
updated-dependencies:
- dependency-name: pdm-project/setup-pdm
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Setup PDM
-        uses: pdm-project/setup-pdm@v3
+        uses: pdm-project/setup-pdm@v4
         with:
           python-version: ${{ matrix.python-version }}
           cache: true