Enable Continuous Integration with GitHub Actions.

This automatically runs unit tests on various operating systems and Python versions when Python files are modified to ensure that functionality remains correct.

.github/workflows/pythonapp.yml

@@ -0,0 +1,31 @@
+name: Test code
+
+on:
+  push:
+    paths:
+    - "**/*.py"
+  pull_request:
+    paths:
+    - "**/*.py"
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-latest, windows-latest]
+        python-version: [3.6, 3.7, 3.8]
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v1
+      with:
+        python-version: ${{ matrix.python-version }}
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install -r requirements.txt
+    - name: Test with unittest
+      run: |
+        python -m unittest discover tests