Minor packaging / hygiene

[kslote1]

• requirements.txt is a raw pip freeze of a dev machine (jupyter, sphinx, appnope, argon2-cffi, …) pinned with ==, and inconsistent with pyproject.toml (numpy~=2.2.6 there vs. numpy>=1.20 in pyproject — and the code doesn't work on NumPy 2).
• No license header in any of the 14 source files. The MLOSS submission checklist asks that licensing terms be "mentioned in each source file".
• causationentropy/__init__.py does from . import core, datasets, tests — the test package is shipped and imported into the public namespace.
• The README Tests badge points at [github.com/kslote1/causationentropy](http://github.com/kslote1/causationentropy), not the org repo.
• PyPI classifier is Development Status :: 3 - Alpha; consider updating if you consider v1.1 publication-ready.