Add GitHub Actions CI workflow

This workflow is based on the one used in the ros-infrastructure org.

.github/workflows/ci.yaml

@@ -0,0 +1,31 @@
+name: osrf_pycommon-ci
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+    build:
+      strategy:
+        matrix:
+          os: [macos-latest, ubuntu-latest, windows-latest]
+          python: ['3.7', '3.8', '3.9', '3.10']
+          include:
+          - os: ubuntu-18.04
+            python: '3.6'
+      name: osrf_pycommon tests
+      runs-on: ${{matrix.os}}
+
+      steps:
+      - uses: actions/checkout@v2
+      - name: Set up Python ${{matrix.python}}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{matrix.python}}
+      - name: Install dependencies
+        run: |
+          python -m pip install -U -e .[test] pytest-cov
+      - name: Run tests
+        run: |
+          python -m pytest tests --cov