fix: bump requires-python to >=3.10 to match py-cid floor

[Santiagocetran]

What

Bumps requires-python in pyproject.toml from >=3.9 to >=3.10.

Why

Flagged during the review of #12. dincli depends on py-cid>=0.5.0 (pyproject.toml, dincli/requirements.txt), but py-cid==0.5.0's own metadata requires Python >=3.10 (verified against the PyPI JSON API — info.requires_python == ">=3.10" — and its classifiers list only Python 3.10–3.14). So the project advertised >=3.9 support it does not actually have: a pip install on Python 3.9 would fail to resolve py-cid.

This is a pre-existing inconsistency unrelated to the din-node containerization work in #12, so it's split out here as its own one-line fix per review guidance.

Scope

• One line in pyproject.toml. No code or behavior change.
• The din-node image already runs Python 3.12, so it is unaffected.

[umeradl]

Good catch from the #12 review. The fix is correct: py-cid==0.5.0 declares requires_python >= '3.10' in its PyPI metadata, so our >=3.9 floor was never achievable. One-line, no behavior change, clean commit message. LGTM.