Publish to PyPI using GitHub Actions

Regular commits are all built and publish to TestPyPI, tagged commits
are published to PyPI.

This should finish the process of moving away from Travis CI, now that
both testing and PyPI deployments are handled in github actions.

.github/workflows/pypi.yml

@@ -0,0 +1,39 @@
+name: pypi
+
+on: push
+
+jobs:
+  build-and-publish:
+    name: Build and publish to PyPI and TestPyPI
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.9
+    - name: Install pypa/build
+      run: >-
+        python -m
+        pip install
+        build
+        --user
+    - name: Build binary wheel and source tarball
+      run: >-
+        python -m
+        build
+        --sdist
+        --wheel
+        --outdir dist/
+        .
+    - name: Publish distribution to TestPyPI
+      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 }}
+