Add Python 3.10 support

[Master0fFate]

Summary

• lower package metadata to Python >=3.10 and add the Python 3.10 classifier
• add a tomllib/tomli fallback so runtime version loading works on Python 3.10
• add a GitHub Actions pytest matrix for Python 3.10, 3.11, 3.12, and 3.13
• update README badges and uv.lock to match the new support floor

Why Python 3.10 matters

Python 3.10 is still a common floor on LTS and enterprise systems, especially Ubuntu 22.04-era environments and managed developer machines. Supporting it lets those users install UncommonRoute without forcing a Python runtime upgrade, while keeping the compatibility delta small because the main blocker was stdlib tomllib, which has a standard tomli backport.

I realize adding another Python version also adds ongoing QA and CI matrix burden. Technically, though, this looks feasible with a narrow compatibility shim.

Test evidence

Local Python version: Python 3.10.6.

Passed locally:

• uv lock --check
• uv run --python 3.10 --extra dev python -m pytest tests/test_version.py -q
• uv run --python 3.10 --extra dev python -m compileall -q uncommon_route tests
• uv run --python 3.10 --extra dev python -m ruff check uncommon_route/version.py tests/test_version.py

Note: a full pytest run on this Windows machine reached 713 passed / 1 skipped after enabling UTF-8 mode, then failed on existing platform-specific assumptions around POSIX file modes and HOME/shell behavior. The added GitHub Actions matrix runs on ubuntu-latest to provide maintainer-visible 3.10 evidence in CI.