Bump ruff/mypy target to 3.11 and fix pyupgrade violations #128

pyproject.toml declares requires-python = ">=3.11" but the tooling targets are pinned to 3.10:

• [tool.ruff] target-version = "py310"
• [tool.mypy] python_version = "3.10"

The doctrine test infrastructure imports tomllib (stdlib since 3.11), and tomllib does not exist in 3.10.

Proposal:

1. Bump [tool.ruff] target-version from "py310" to "py311"
2. Bump [tool.mypy] python_version from "3.10" to "3.11"
3. Fix the resulting UP017 violations: timezone.utc → datetime.UTC (3.11+)
4. Fix the resulting UP042 violations: (str, Enum) → StrEnum (3.11+)