Solve TypeError for py39

.github/workflows/ci.yml

@@ -37,3 +37,31 @@ jobs:
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./docs/_build
+
+  pytest_plain:
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.9", "3.10", "3.11"]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repo
+        uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e .[all]
+          python -m pip install pytest
+          python -m pip install dask pymetis
+      - name: list env contents
+        run: |
+          conda info
+          conda list
+          pip list
+      - name: Test with pytest
+        run: |
+          pytest

xugrid/ugrid/interpolate.py

@@ -1,3 +1,6 @@
+# to prevent "TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'" in py39
+from __future__ import annotations
+
 import warnings
 
 import numpy as np