Build the python release artifacts in GHA too

.github/workflows/debs.yml → .github/workflows/release-artifacts.yml

-# GitHub actions workflow which builds the debian packages.
+# GitHub actions workflow which builds the release artifacts.
 
-name: Debs
+name: Build release artifacts
 
 on:
   push:
+    # we build on develop and release branches to (hopefully) get early warning
+    # of things breaking
     branches: ["develop", "release-*"]
 
+    # we also rebuild on tags, so that we can be sure of picking the artifacts
+    # from the right tag.
+    tags: ["v*"]
+
 permissions:
   contents: read
 
@@ -40,5 +46,19 @@ jobs:
       - run: ./src/scripts-dev/build_debian_packages "${{ matrix.distro }}"
       - uses: actions/upload-artifact@v2
         with:
-          name: packages
+          name: debs
           path: debs/*
+
+  build-sdist:
+    name: "Build pypi distribution files"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+      - run: pip install wheel
+      - run: |
+          python setup.py sdist bdist_wheel
+      - uses: actions/upload-artifact@v2
+        with:
+          name: python-dist
+          path: dist/*