ci: set Python version for functional tests more specifically

[rossiam]

Specify Python version more specifically so we don't get random newer untested versions trying to run functional tests.

Checklist

• [x] I have read the CONTRIBUTING document
• [x] My code follows the code style of this project (npm run lint produces no warnings/errors)

[changeset-bot[bot]]

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Latest commit: 6329e7c3f8db5774e6906a278afc423251e1d606

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