Work-calendar

1. Went with a cleaner final API. No need to import the WorkCalendar class now to use its methods.
2. Simplified project structure (could it be simpler?)
3. Cleaned up unit tests

What changed

1. Added the latest available info for official workdays calendar for year 2025.
2. Published package to [PyPi](https://pypi.org/project/work-calendar/).
3. Changed the only method in the API from `WorkCalendar.is_day_off` to `WorkCalendar.is_workday`. No need to bump major version though at this point.