fix: restore custom py version

Author: adbar

.github/workflows/tests.yml

@@ -25,9 +25,9 @@ jobs:
         include:
         # custom python versions
         - os: macos-latest
-          python-version: ${{ env.REF_PY_VERSION }}
+          python-version: "3.13"
         - os: windows-latest
-          python-version: ${{ env.REF_PY_VERSION }}
+          python-version: "3.13"
     steps:
     - uses: actions/checkout@v5