Specify exact version in GitHub Action step (#126)

This will hopefully fix a failure that blocked a release yesterday.

Author: mworion

.github/workflows/test_package.yml

@@ -231,7 +231,7 @@ jobs:
     # chosen for the PyPI token.
 
     - name: upload_to_pypi
-      uses: pypa/gh-action-pypi-publish@v1
+      uses: pypa/gh-action-pypi-publish@v1.8.10
       with:
         user: __token__
         password: ${{ secrets.pypi_password }}