Hijriconverter

In short, this version supports only lunar Hijri calendar on Python 3.6+ with
a more accurate conversion in complete alignment with the Umm al-Qura official
calendar.

Details are as follows:

- Dropped support for Solar Hijri calendar.
- Dropped support for Python 3.5.
- Refactored localization and ``ummalqura.py`` module.
- Updated ``month_starts`` tuple to make the conversion more accurate.
- Added ``fromdate()`` classmethod to ``Gregorian`` object.
- Added ``notation()`` method to ``Hijri`` and ``Gregorian`` objects.
- Added more methods to ``Gregorian`` object including ``slashformat()``,
``month_name()``, ``day_name()`` and ``to_julian()``.
- Renamed ``month_days()`` method of ``Hijri`` object to ``month_length()``.
- Changed formatted string to use f-strings.
- Improved documentation and examples.
- Updated unit tests.
- Fixed other minor issues and typos.

- Added ``fromisoformat()`` classmethod to ``Hijri`` object.
- Added support for rich comparison between Hijri dates.
- Updated documentation and testing code.
- Other minor fixes and enhancements.

- Refactored conversion methods to improve performance.
- Changed date validation back to be the default and removed optional parameter.
- Added ``to_julian()`` method to ``Hijri`` object.
- Updated documentation and testing code.
- Other minor fixes and enhancements.

- Fixed a bug in range validation for the Gregorian date.
- Changed generic typing to built-in types.
- Added more tests to cover the solar calendar.
- Improved code structure and documentation.

- Improved documentation and changelog.

- Fixed README file.