COMP: For windows-2019 Azure Pipelines CI, use Python version 3.9

It appears that windows-2019 still has Python 3.7.9 by default, which is no longer supported by SimpleITK.

- See also pull request #989 commit ea765f2 "COMP: Upgrade Azure Pipelines CI from windows-2019 to windows-2022"

yml task code copied from https://github.com/InsightSoftwareConsortium/ITK/blob/69738a9d64f46e0f555d3cfc4bf7d01de1e4178a/Testing/ContinuousIntegration/AzurePipelinesWindows.yml#L44-L47

Testing/CI/Azure/ci.yml

@@ -74,6 +74,10 @@ jobs:
   - bash: ../../../externalproject-build/Release/elastix_translation_example
     displayName: 'Run externalproject'
     workingDirectory: $(Agent.BuildDirectory)\Elastix-build\bin\Release
+  - task: UsePythonVersion@0
+    inputs:
+      versionSpec: '3.9'
+      architecture: 'x64'
   - bash: python -m pip install -r "$(ELASTIX_SOURCE_DIR)/Testing/PythonTests/requirements.txt"
     displayName: 'Install PythonTests requirements'
   - bash: ctest -C Release -VV -j 2 -E "elastix_run_example_COMPARE_IM|elastix_run_3DCT_lung.MI.bspline.ASGD.001_COMPARE_TP"