Fix Python version requirement to match CI test matrix

Changed requires-python from ">=3.12" to ">=3.9" to match the Python
versions tested in CI (3.9, 3.10, 3.11, 3.12).

The >=3.12 requirement was preventing quippy from building on Python 3.11,
which caused the f90wrap QUIP regression test to fail.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jameskermode

quippy/pyproject.toml

@@ -12,7 +12,7 @@ name = "quippy-ase"
 version = "0.9.0"
 description = "ASE-compatible Python bindings for the QUIP and GAP codes"
 readme = {file = "README", content-type = "text/plain"}
-requires-python = ">=3.12"
+requires-python = ">=3.9"
 dependencies = [
     "numpy>=1.20",
     "ase>=3.17.0",