Run the test suite with GitHub Actions

[rodvals]

Dependency

This pull request is stacked on #8 and should be merged after it.

Summary

• run the test suite for pushes to master and pull requests targeting master
• create a Python 3.12 Miniforge environment with graph-tool from conda-forge
• install CPU-only torch and the project development dependencies with pip
• restrict workflow permissions to repository read access
• cancel superseded workflow runs for the same branch
• stop the job after 30 minutes if installation or tests become stuck
• reject unknown pytest configuration and markers
• report every skipped test and its reason