Workalendar

Bugfixes

- Belarus: removing day after Radonitsa, which is apparently not a holiday.
- Algeria: assigning the week-end days as FRI+SAT, as it's following a Islamic calendar.

Other changes

- Refactoring the core ``Calendar`` classes / mixins for better understanding. Only one ``Calendar`` subclass should be imported / used in calendar classes, the rest (when possible) should be ``Mixins`` (related to 511).
- Declaring the New year's Day as a worldwide holiday, with only two exceptions (to date): Israel & Qatar (511).
- Fixed `contributing.md` documentation with the new class/mixin organization (511).

- Bugfix: setting *Consciência Negra day* as a non-holiday by default for Brazilian calendars, thx to edniemeyer (516).
- Bugfix: Introducing the changes in Croatia holidays as of 2020 - Remembrance Day, Independence Day, Statehood Day... thx to davidpodrebarac for the bug report (515).

Calendar fix

- Adding All Souls' Day to Lithuania calendar, starting of 2020, thx to norkunas (512).

Minor changes

- Small fixes (docstrings, use of extends, etc) on Cayman Islands calendar (507).
- Moving Carnaval / Mardi Gras / Fat Tuesday calculation into the `workalendar.core` module, because it's used in at least 3 countries and some States / Counties in the USA.

- **BREAKING CHANGE**: the ``IsoRegistry.get_calendar_class()`` method has been removed from the code and should no longer be used (375, 495).

New Calendars

- Added rules for all Switzerland Cantons, branching off the initial work by brutasse (497).

- Making the Israel calendar more efficient (498).
- Fixing duplicated holidays in Hong-Kong and Hong-Kong Bank holiday calendars (496).
- Integrating Hong-Kong holidays for 2021 (496).