build: add GH workflow to build + release on PyPI

Add workflow with two jobs to build and publish on PyPI.  The
release job waits for the build job and uses a custom release
environment, which can be configured to require review.

To share the build artifacts between the jobs and to make them
available for intermediate review, they are stored using
'actions/upload-artifact' and 'actions/download-artifact'.
https://docs.github.com/en/actions/using-workflows/storing-workflow-data-as-artifacts

To upload the build artifacts to PyPI, the PyPA recommended
'pypa/gh-action-pypi-publish' is used.
https://packaging.python.org/en/latest/guides/publishing-package-distribution-releases-using-github-actions-ci-cd-workflows/

**Caveat**
The URL to grab the artifacts, e.g. for review, requires knowledge
of action ID and artifact ID, and a login token (no special
permissions). This makes it a bit cumbersome to fetch the artifacts
with a script and compare them to a local build.
https://docs.github.com/en/actions/managing-workflow-runs/downloading-workflow-artifacts

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

.github/workflows/cd.yml

@@ -0,0 +1,51 @@
+name: CD
+concurrency: cd
+
+# Trigger workflow on release tag push
+on:
+  push:
+    # TODO: Should we restrict to vX.Y.Z tags?
+    tags: v*
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout release tag
+        uses: actions/checkout@a12a3943b4bdde767164f792f33f40b04645d846
+
+      - name: Set up Python
+        uses: actions/setup-python@0ebf233433c08fb9061af664d501c3f3ff0e9e20
+        with:
+          python-version: '3.x'
+
+      - name: Install build dependency
+        run: python3 -m pip install --upgrade pip build
+
+      - name: Build binary wheel and source tarball
+        run: python3 -m build --sdist --wheel --outdir dist/ .
+
+      - name: Store build artifacts for review and release
+        uses: actions/upload-artifact@6673cd052c4cd6fcf4b4e6e60ea986c889389535
+        with:
+          name: build-artifacts
+          path: dist
+
+  release-on-pypi:
+    name: Release on PyPI
+    runs-on: ubuntu-latest
+    needs: build
+    environment: release
+    steps:
+      - name: Fetch build artifacts
+        uses: actions/download-artifact@fb598a63ae348fa914e94cd0ff38f362e927b741
+        with:
+          name: build-artifacts
+          path: dist
+
+      - name: Publish binary wheel and source tarball on PyPI
+        uses: pypa/gh-action-pypi-publish@717ba43cfbb0387f6ce311b169a825772f54d295
+        with:
+          user: __token__
+          password: ${{ secrets.PYPI_API_TOKEN }}