Enable usage of pre-releases for test pypi

Author: blowekamp

.github/workflows/main.yml

@@ -82,7 +82,7 @@ jobs:
         jupyter nbextension enable --py --sys-prefix widgetsnbextension
     - name: Install SimpleITK from test PyPi from manually launched workflow
       if: ${{ inputs.testPyPi }}
-      run: pip install --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple SimpleITK
+      run: pip install --pre --upgrade --index-url https://test.pypi.org/simple/ --extra-index-url https://pypi.org/simple SimpleITK
     - name: Download data
       if: steps.cache.outputs.cache-hit != 'true'
       run: python Utilities/downloaddata.py Data/ Data/manifest.json