Add poetry and update CI

[ElliottKasoar]

• Replaces setup.py/requirements.txt with dependencies in pyprpject.toml and poetry
• Updates dependency versions (includes deprecating Python 3.8, as unsupported e.g. by matplotlib)
• Replaces travis CI with GitHub Actions
  • Currently does not include documentation building, but this is very limited and needs updating

Tests currently passing: https://github.com/ElliottKasoar/abcd/actions/runs/9465590767