Update CI and migrate to pyproject.toml

[Sebastian-Belkner]

Changelog

• Replaced deprecated setup.py with pyproject.toml
• All dependencies now explicitly declared, including healpy, astropy, ducc0 and scipy which were previously missing
• Tested on Python 3.9–3.12, NumPy pre- and post-2.0
• Added multi-Python install matrix CI (install.yaml)
• Added end-to-end smoke test CI (smoke_test.yaml) — runs sim generation, QE, and MAP reconstruction on a minimal config
• Updated README

[Sebastian-Belkner]

@julientang I've updated the installation, I hope that things work more smoothly for you now

Please let me know if things now work as expected - CI tests 3.19 to 3.12 and numpy pre2.0 and post2.0 and appears to be working