Make scipy optional again and declare packaging

[Kayvan-Zahiri]

On a clean install of seekpath 2.2.1 (no bz extra), import seekpath fails:

$ pip install seekpath
$ python -c "import seekpath"
ModuleNotFoundError: No module named 'scipy'

scipy is listed only under the bz extra, and the tests already skip the Brillouin zone tests when it's missing, but seekpath/__init__.py has imported brillouinzone since cffa9d5 (March 2024), and that module imports scipy.spatial at the top.

Two changes:

• brillouinzone.py imports scipy.spatial inside BZ._initialize_BZ, the only place it's used at runtime. The return annotations on hull and delaunay stay as they are through from __future__ import annotations and a TYPE_CHECKING import. Building a BZ without scipy still raises ModuleNotFoundError: No module named 'scipy'.
• With that fixed, get_path hit a second missing dependency: hpkot/tools.py imports packaging, which isn't declared. I added it to dependencies.

Tested: in a clean Python 3.12 venv with a plain install (no scipy), import seekpath and seekpath.get_path(...) work on this branch; on 2.2.1 the import fails. With .[bz] installed, the full test suite passes (77 tests, including the new tests/test_import_without_scipy.py, which blocks scipy in a subprocess and imports seekpath; it fails without this change). The trailing-whitespace, end-of-file, quote-fixer, flynt and ruff hooks pass on the changed files.

Written with AI assistance. The runs above are real.

[t-reents]

Thanks!
However, while working on #124, I noticed this issue myself and fixed it locally. I now opened a PR #125, which will supersede this one, so I'll close it.
Nonetheless, thank you for your contribution.