Declare us statically as not-compatible with Python 3.14

[mwaskom]

Currently using the modal SDK on an unsupported new Python version raises a noisy runtime error. We can do better by including the upper bound in our pyproject.toml metadata.

In practice, I think that users on 3.14 will just get the oldest version of the client without this change. But it sets a precedent that we can follow in the future.

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Note

Sets an upper bound on supported Python versions (<3.14) in project metadata.

• Project metadata:
  • Update pyproject.toml requires-python to >=3.9,<3.14 to declare incompatibility with Python 3.14.

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