Modernizes python-datemath to a current 2026 Python project layout and removes the remaining legacy install paths.

This MR replaces setup.py/setup.cfg/requirements.txt with a single pyproject.toml, adopts uv for local development and builds, migrates the old unittest file into a pytest test suite, and adds centralized ruff and mypy configuration. CI and release workflows were updated to use current GitHub Actions plus uv, and the Docker image now builds from a current Python base image instead of the old Python 3.9 + setup.py flow.

It also includes a small cleanup pass in the library itself: the unit alias handling is simpler, timezone parsing is a bit more robust with current Arrow/Python behavior, and the tests now use stdlib zoneinfo rather than pytz.

Validation performed:

uv run pytest
uv run ruff check .
uv run mypy
uv build
uv run twine check dist/*
uv run --with pip-audit pip-audit
Main changes:

add pyproject.toml and uv.lock
remove legacy packaging files: setup.py, setup.cfg, requirements.txt
migrate tests from tests.py to tests/test_datemath.py
modernize CI/release workflows in .github/workflows
update Dockerfile and add .dockerignore
Notes:

minimum supported Python version is now 3.11
test coverage was migrated and hardened, not replaced wholesale
pip-audit reported no known Python package vulnerabilities after the migration

Add releaserc
Add pre-commit