Raise the Python floor to 3.10 and modernize packaging #29

The packages declare requires-python >= 3.8, but the codegen already needs 3.10, Python 3.8 and 3.9 are end of life, and the generated types use modern typing.

Follow-up:

• Set requires-python >= 3.10 in all three packages and the root, and update the classifiers and the CI matrix.
• Move to a uv or hatch based workflow with a lock file, so contributors and CI install the same versions (the ruff drift fixed in chore: regenerate SDK from current public API schema #20 came from unpinned dev dependencies).
• Pin dev tools (ruff, mypy, pytest) to exact versions.