Add basic CI pipeline

[elliot-100]

Use GitHub Actions to automate build and basic verification (there are no tests yet) on relevant Python versions.

[elliot-100]

Note that this PR is from my personal fork.
This was to verify it runs OK on branch named main.
See elliot-100#1 and https://github.com/elliot-100/Spond/actions/runs/3884988324

• Takes < 60 s, but I think can be speeded up a little by not installing all dev dependencies.

• Can be extended to add linting/style enforcement, publish to PyPI etc in future...