Add GitHub Actions testing workflow (#24)

* Add GitHub Actions testing workflow

* Update named Python versions and dev status

* Use unittest.mock as default mock source

Fall back to package for Python 2.7

* Try installing mock outside of setup

* Switch to GitHub actions shield

.github/workflows/test.yml

@@ -0,0 +1,23 @@
+name: Tests
+
+on: [push]
+
+jobs:
+  test:
+
+    name: Test on Python ${{ matrix.py_version }}
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        py_version: [2.7, 3.5, 3.6, 3.7]
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python ${{ matrix.py_version }}
+      uses: actions/setup-python@v1
+      with:
+        python-version: ${{ matrix.py_version }}
+    - name: Install mock for Python 2.7
+      run: pip install mock
+    - name: Run tests
+      run: python setup.py test