Calendra

- Added DEEPAVALI days for 2019 and 2020, thx pvalenti (282).
- Fixed Germany Reformation Day miscalculation. Some German states include Reformation Day since the "beginning" ; in 2017, all states included Reformation Day as a holiday (500th anniversary of the Reformation) ; starting of 2018, 4 states added Reformation Day (295).

- Removed dependency to `PyEphem`. This package was the "Python2-compatible" library to deal with the xephem system library. Now it's obsolete, so you don't need this dual-dependency handling, because `ephem` is compatible with Python 2 & Python 3 (296).
- Raise an exception when trying to use unsupported date/datetime types. Workalendar now only supports stdlib `date` & `datetime` (sub)types. See the `basic documentation <https://workalendar.github.io/workalendar/basic.html#standard-datetime-types-only-please>`_ for more details (294).

- Fixed ISO 3166-1 code for the `Slovenia` calendar (291, thx john-sandall).

- Added support for Python 3.7 (283).
- Fixed the `SouthAfrica` holidays calendar, taking into account the specs of holidays that vary over the periods. As a consequence, it cleaned up erroneous holidays that were duplicated in some years (285). Thx to surfer190 for his review & suggestions.
- Bugfix for South Africa: disabled the possibility to compute holidays prior to the year 1910.
- Renamed Madagascar test class name into `MadagascarTest` (286).
- Separated the coverage jobs from the pure tests. Their report output was disturbing in development mode, you had to scroll your way up to find eventual failing tests (289).

===

Large work on global registry: refs (13), (96), (257) & (284).

- Added Tests for Europe registry.
- Revamped and cleaned up Europe countries.
- Added the United States of America + States, American countries & sub-regions, African countries, Asian countries, Oceanian countries.
- The global registry usage is documented.
- Changed Canada namespace to `workalendar.america.canada`.
- You don't have to declare a `name` properties for Calendar classes. It will be deducted from the docstring.
- Changed the `registry.items()` mandatory argument name to `region_codes` for more readability.