fix: build package distribution

[ceberam]

• set project license according to PEP 639
• sync the environment after updating the package version and ensure that uv.lock is commited
• add missing package files
• replace setuptools with hatchling as build system

[mergify]

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[vagenas]

@ceberam looks good, just can you provide some context to the switch from setuptools to hatchling?

[ceberam]

@ceberam looks good, just can you provide some context to the switch from setuptools to hatchling?

After finding out that the first distribution was built incorrectly, I was not able to configure the setuptools options to produce the right build for our flat-layout. Neither with automatic discovery nor with the find directive. It may be related to the uv build front end that we use in this repository.
The uv tool suggests hatchling by default and other apps like pytest also point at that build system. With hatchling I was able to run the uv build and get the source and binary distributions as expected.