Relax Python version requirement from >=3.13 to >=3.10

[MaxGhenis]

Problem

pyproject.toml currently requires >=3.13,<3.14, but this appears to be unintentional:

1. The old setup.py had python_requires=">=3.7"
2. policyengine-us uses >=3.10,<3.14
3. policyengine-core uses >=3.10
4. The classifiers in pyproject.toml list Python 3.10, 3.11, 3.12, 3.13

When pyproject.toml was introduced in PR #1395 (Nov 2025), it was set to >=3.13 rather than matching the previous >=3.7 or aligning with other PE packages.

Impact

Users on Python 3.10-3.12 get a confusing pip error:

ERROR: Could not find a version that satisfies the requirement policyengine-uk==2.72.3

This particularly affects Modal deployments and other environments that default to Python 3.11.

Proposed Fix

Change in pyproject.toml:

requires-python = ">=3.10,<3.14"

This aligns with policyengine-us and should work unless there's a specific 3.13+ dependency I'm not aware of.