Add GitHub Actions workflow for CI

.editorconfig

@@ -8,3 +8,7 @@ insert_final_newline = true
 [*.rst]
 indent_style = space
 indent_size = 4
+
+[*.yml]
+indent_style = space
+indent_size = 2

.github/workflows/ci.yml

@@ -0,0 +1,21 @@
+on: push
+jobs:
+  test:
+    strategy:
+      matrix:
+        platform: [macos-latest, ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.platform }}
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions/setup-python@v1
+        with:
+          python-version: "3.6"
+      - uses: actions/setup-python@v1
+        with:
+          python-version: "3.7"
+      - uses: actions/setup-python@v1
+        with:
+          python-version: "3.8"
+      - run: python -m pip install --upgrade pip
+      - run: python -m pip install --upgrade tox
+      - run: tox

tox.ini

@@ -1,4 +1,5 @@
 [tox]
+# When adding a new Python version here, please also update the list of Python versions called by the GitHub Actions workflow (.github/workflows/ci.yml).
 envlist = py{36,37,38},mypy
 
 [testenv]