Add action

- Add GitHub unit tests action

.github/workflows/unittest.yml

@@ -0,0 +1,30 @@
+name: md-toc
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        # 3.13 is still in alpha.
+        python-version: ['3.8', '3.9', '3.10', '3.11', '3.12']
+        os:  ['ubuntu-latest', 'macos-latest', 'windows-latest']
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      # You can test your matrix by printing the current Python version
+      - name: Display Python version
+        run: python -c "import sys; print('Python version = ' + str(sys.version))"
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install --requirement requirements.txt --requirement requirements-dev.txt
+      - name: Run unit tests
+        run: python -m unittest discover --failfast --locals --verbose