Add github action.

.github/workflows/tests.yml

@@ -0,0 +1,28 @@
+name: Python package
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install -U pip coverage codecov flake8 fastimport
+    - name: Style checks
+      run: |
+        make style
+    - name: Coverage test suite run
+      run: |
+        python -m coverage run -p -m unittest dulwich.tests.test_suite