chore: drop Python 3.10 support

[tabedzki]

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What

Raises the Python floor from 3.10 to 3.11.

• pyproject.toml: requires-python = ">=3.11", remove the 3.10 classifier
• uv.lock: re-locked with a plain uv lock (no --upgrade) — drops all python_full_version < '3.11' markers, the py3.10 wheels, and the tomli backport (stdlib tomllib since 3.11). No package versions were bumped.
• CI (tests.yml): fix the Codecov upload if: gate that was pinned to '3.10' — which never matched the ['3.12'] matrix, so coverage upload was silently dead. Now gated on '3.12'.
• Docs / README / tests README: state Python 3.11+ instead of 3.10+

Why

Python 3.10 reaches end-of-life in October 2026. Dropping it now unblocks moving to dependencies that require >=3.11 — notably pandas 3.0, which dropped 3.10 support. (The pandas 3.0 floor bump itself lives in the separate chained-assignment PR #39; this PR only removes the 3.10 constraint that would otherwise make pandas>=3.0 unsatisfiable.)

Verification

• Full test suite passes: 120 passed (Python 3.14, pandas 2.3.1).
• uv lock --check passes; lock diff is pure marker/wheel removal (−698 lines), no upgrades.
• Repo swept for 3.10-era compatibility shims (sys.version_info guards, typing_extensions, tomli imports) — none in source, so no code changes were needed.

Note

This is intentionally scoped to the version drop. It pairs with #39 (pandas 3.0 chained-assignment fix); once both merge, a follow-up can raise the constraint to pandas>=3.0 now that the floor allows it.