Add GitHub Actions test config

Run a GitHub Action to run the existing test suite any time commits are
pushed to the `master` branch or a pull request targets it. Also include
an initial test matrix lay groundwork for future additional Python
version support.

.github/workflows/test.yml

@@ -0,0 +1,30 @@
+name: Test
+
+on:
+  push:
+    branches:
+      - "master"
+  pull_request:
+    branches:
+      - "master"
+
+jobs:
+  tests:
+    name: "Python ${{ matrix.python-version }}"
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version:
+          - "3.6"
+
+    steps:
+      - uses: "actions/checkout@v2"
+      - uses: "actions/setup-python@v2"
+        with:
+          python-version: "${{ matrix.python-version }}"
+
+      - name: "Run tests"
+        run: |
+          python -m pip install pipenv
+          pipenv --python python3.6 install --dev
+          pipenv run pytest tests