ci(python-deps): update transformers version constraints in tox

[liferoad]

Fixes #30799 (comment)

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[gemini-code-assist]

Summary of Changes

Hello @liferoad, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the Python SDK's testing environment by introducing conditional version constraints for the transformers library within tox.ini. This change specifically targets compatibility issues related to Python's union type syntax across different Python versions, ensuring stable test execution. Additionally, it enhances debugging by logging the installed transformers version during CI runs.

Highlights

• Transformers Version Constraints: Updated transformers dependency constraints in sdks/python/tox.ini to be Python version-specific, addressing union type syntax issues for different Python versions.
• Debugging Aid: Added a pip freeze | grep -E transformers command to the tox.ini test commands to help with debugging the installed transformers dependency version during CI runs.
• CI Trigger: A minor, trivial change was made to a CI trigger file (.github/trigger_files/beam_PostCommit_Python_Dependency.json) to ensure the relevant Python dependency tests are re-run.

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[liferoad]

https://github.com/apache/beam/actions/runs/18501629474/job/52719753121?pr=36506

We need more than 3 hours for this workflow now.

[liferoad]

https://github.com/apache/beam/actions/runs/18508300753/job/52759708202 is green now.