- Dropped support for Python 3.5.
- Added `fromstring()` classmethod to allow constructing the `Week` object from
a formatted string, for example '2019W08' or '201908'.
- Added `cdcformat()` method to return a formatted string like the one used by
US CDC for epi weeks, for example for week 8 of 2019 it returns '201908'.
- Added `daydate()` method to return the date of specific weekday for a week.
- Changed the optional value 'WHO' of the parameter `method` to 'ISO'.
- Changed the `fromdate()` classmethod of `Week` object to accept a date object
as an argument instead of year, month, and day.
- Made the `Week` and `Year` objects hashable.
- Improved rich comparison methods.
- Improved input data validation.
- Updated and improved documentation examples.
- Updated unit tests.
- Other minor fixes and enhancements.