add action to run pytests (#1167)

This PR adds a github action that runs pytests on the python code within pygw.
The package is tested with python versions 3.7,. 3.8, 3.9 and 3.10.
After this PR, tests will be required with every new feature added to `pygw`.

.github/workflows/pytests.yaml

@@ -0,0 +1,35 @@
+name: Run pytests
+on: [push, pull_request]
+
+jobs:
+  run_pytests:
+    runs-on: ubuntu-latest
+    name: Install pygw and run tests with pytests
+    strategy:
+      matrix:
+        python: ["3.7", "3.8", "3.9", "3.10"]
+
+    steps:
+      - name: Setup Python
+        uses: actions/setup-python@v3
+        with:
+          python-version: ${{ matrix.python }}
+
+      - name: Install (upgrade) dependencies
+        run: |
+          pip install --upgrade pip
+
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          path: global-workflow
+
+      - name: Install pygw
+        run: |
+          cd $GITHUB_WORKSPACE/global-workflow/ush/python/pygw
+          pip install .[dev]
+
+      - name: Run pytests
+        run: |
+          cd $GITHUB_WORKSPACE/global-workflow/ush/python/pygw
+          pytest -v src/tests