build: Use importlib.metadata for versioning

[nvictus]

Version determination

This PR changes the way __version__ is set.

Old way:

• build time: dynamic build-time determination of version by reading the __variable__ from a source file
• run time: __version__ is specified in the source code

New way:

• build time: we now specify the package version statically in pyproject.toml
• run time: use the import system importlib.metadata.version to retrieve the appropriate version number from installed package metadata

This method is preferred because it ensures that the version information is centralized in one place (in the package distribution metadata) and reduces the risk of inconsistencies between the code and the package metadata.

Other changes

• Make all __all__ exports fully explicit.
• Update ruff invocation syntax and configuration.
• Add a hatch test env matrix for multiple Python versions.