Add package build step to actions

h/t to Hynek as this is stolen wholesale from structlog's Actions
configuration.

.github/workflows/main.yml

@@ -39,4 +39,25 @@ jobs:
         run: |
           tox -e coverage
 
-      # TODO: get codecov working
+      # TODO: get codecov working
+
+  package:
+    name: "Build & verify package"
+    runs-on: "ubuntu-latest"
+
+    steps:
+      - uses: "actions/checkout@v2"
+      - uses: "actions/setup-python@v2"
+        with:
+          python-version: "3.10"
+
+      - name: "Install build, check-wheel-contents, and twine"
+        run: "python -m pip install build twine check-wheel-contents"
+      - name: "Build package"
+        run: "python -m build --sdist --wheel ."
+      - name: "List result"
+        run: "ls -l dist"
+      - name: "Check wheel contents"
+        run: "check-wheel-contents dist/*.whl"
+      - name: "Check long_description"
+        run: "python -m twine check dist/*"