Pin python version to 3.12

[1pav]

According to pyproject.toml the minimum required python version for the project is 3.6:

requires-python = ">=3.6"

however, uv will pick the first python version that is compatible with the requirement. On my environment (F43), that is python3.14, which seems to be incompatible with some code, e.g. when running tests:

________________________________________________________ ERROR collecting tests/test_registry_query.py _________________________________________________________
tests/test_registry_query.py:8: in <module>
    from flatpak_indexer.registry_query import query_registry_image
flatpak_indexer/registry_query.py:4: in <module>
    from .models import ImageModel
flatpak_indexer/models.py:21: in <module>
    class ImageModel(BaseModel):
flatpak_indexer/json_model.py:277: in __new__
    k: _make_model_field(k, v, getattr(x, k, None)) for k, v in annotations.items()
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
flatpak_indexer/json_model.py:267: in _make_model_field
    raise TypeError(f"{name}: Unsupported type {resolved}")
E   TypeError: pull_spec: Unsupported type <class 'typing.Union'>

The above will work fine with python3.12.

It is usually recommended to pin python version via .python-version. Apparently we are using 3.12 also in CI, so that's what I set it to.

[gemini-code-assist]

Summary of Changes

Hello @1pav, 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 addresses a compatibility issue where the project's dependency management tool (uv) could select a Python version newer than 3.12, leading to runtime errors, specifically TypeError with typing.Union. By introducing a .python-version file, the project now explicitly specifies Python 3.12, ensuring a consistent and stable development and testing environment that matches the CI setup.

Highlights

• Python Version Pinning: The project's Python version has been explicitly pinned to 3.12 to resolve compatibility issues encountered with newer Python versions (e.g., 3.14) that caused TypeError exceptions during testing.
• Environment Consistency: A .python-version file has been introduced, setting the Python version to 3.12, aligning the local development environment with the version already used in CI.

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[gemini-code-assist]

Code Review

This pull request pins the Python version to 3.12 by adding a .python-version file, which is a good measure to ensure development environment consistency and resolve the noted incompatibility with Python 3.14. My main feedback is that the requires-python setting in pyproject.toml should also be updated to reflect this stricter version requirement. This will ensure the package's metadata is accurate and prevent installation on unsupported Python versions.

[owtaylor]

Let's update the .python-version in pyproject.toml to 3.12 but leave it as a >=

[owtaylor]

LGTM