workflows: generate sdist tarball in CI

By omitting the `python -m build` -w argument in one job, verify that the
source tarball can successfully build a wheel.  Use that source tarball in
releases rather than building it on a developer's machine.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>

.github/ISSUE_TEMPLATE/release.md

@@ -5,7 +5,6 @@
 - [ ] `git clean -dxf && mkdir dist`
 - [ ] Find the [workflow run](https://github.com/openslide/openslide-python/actions) for the tag; download its docs and wheels artifacts
 - [ ] `unzip /path/to/downloaded/openslide-python-wheels.zip && mv openslide-python-wheels-*/* dist/`
-- [ ] `python setup.py sdist`
 - [ ] `twine upload dist/*`
 - [ ] Recompress tarball with `xz`
 - [ ] Attach release notes to [GitHub release](https://github.com/openslide/openslide-python/releases/new); upload tarballs and wheels

.github/workflows/python.yml

@@ -36,6 +36,9 @@ jobs:
         # last upstream patch releases that ship with installers.
         # https://github.com/actions/setup-python/issues/439#issuecomment-1247646682
         include:
+          - os: ubuntu-latest
+            python-version: "3.12"
+            sdist: sdist
           - os: macos-latest
             python-version: "3.9"
             upstream-python: 3.9.13
@@ -82,7 +85,10 @@ jobs:
         esac
     - name: Build wheel
       run: |
-        python -m build -w
+        if [ -z "${{ matrix.sdist }}" ]; then
+            wheel_only=-w
+        fi
+        python -m build $wheel_only
         case "${{ matrix.os }}" in
         ubuntu-*)
             mkdir old