fix pyproject.toml: The Poetry configuration is invalid

[wirthual]

Running poetry install results in this error:

The Poetry configuration is invalid:
  - Additional properties are not allowed ('python-version' was unexpected)

With this change I can successfully install the package:

poetry install
Installing dependencies from lock file
No dependencies to install or update
Installing the current project: gemma (1.0.0)

Poetry version 1.7.1

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