pin Python version to 3.8 for dependabot

[gruebel]

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Completion checklist

• Additions and changes have unit tests
• The pull request has been appropriately labeled using the provided PR labels
• GitHub actions automation is passing (make test, make lint, make security-test, make test-js)
• If the UI contents or JavaScript files have been modified, generate a new example report:

# Generate the updated Javascript bundle
make build-js

# Generate the example report
make generate-report