Try only uploading source for 3.8

The matrix means every step is run for every version, but the source tar
can only be uploaded once. Not sure if this is the right syntax.

Author: polm

.github/workflows/linux.yml

@@ -24,7 +24,7 @@ jobs:
       run: |
         pytest
     - name: Publish to PyPI if tagged
-      if: startsWith(github.ref, 'refs/tags')
+      if: startsWith(github.ref, 'refs/tags') && matrix.python-version == 3.8
       env:
         TWINE_USERNAME: "23"
         TWINE_PASSWORD: ${{ secrets.PYPI_PASSWORD }}