Switch to GitHub Actions

Travis has imposed quotas on builds that require us to contact them to
obtain OSS credits. The CI tool is irrelevant so long as builds do
happen, so switch to GitHub Actions for now.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

.github/workflows/ci.yaml

@@ -0,0 +1,88 @@
+---
+name: CI
+on:
+  - push
+  - pull_request
+jobs:
+  lint:
+    name: Run linters
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v2
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - name: Install dependencies
+        run: python -m pip install tox
+      - name: Run tox
+        run: tox -e pep8,mypy
+  test:
+    name: Run unit tests
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python: [3.6, 3.7, 3.8, 3.9]
+    steps:
+      - name: Checkout source code
+        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 tox
+      - name: Run unit tests (via tox)
+        # Run tox using the version of Python in `PATH`
+        run: tox -e py
+  docs:
+    name: Build docs
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - name: Install dependencies
+        run: python -m pip install tox
+      - name: Build docs (via tox)
+        run: tox -e docs
+      - name: Archive build results
+        uses: actions/upload-artifact@v2
+        with:
+          name: html-docs-build
+          path: docs/_build/html
+          retention-days: 7
+  release:
+    name: Upload release artifacts
+    runs-on: ubuntu-latest
+    needs: test
+    if: github.event_name == 'push'
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - name: Install dependencies
+        run: python -m pip install build
+      - name: Build a binary wheel and a source tarball
+        run: python -m build --sdist --wheel --outdir dist/ .
+      - name: Publish distribution to Test PyPI
+        uses: pypa/gh-action-pypi-publish@master
+        with:
+          password: ${{ secrets.TEST_PYPI_API_TOKEN }}
+          repository_url: https://test.pypi.org/legacy/
+      - name: Publish distribution to PyPI
+        if: startsWith(github.ref, 'refs/tags')
+        uses: pypa/gh-action-pypi-publish@master
+        with:
+          password: ${{ secrets.PYPI_API_TOKEN }}