Convertdate

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* Drop official support for Python version 3.5 and 3.6, which have reached end-of-life.
* Remove pytz dependency (49)

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* Switch Persian calendar to astronomical computation (was non-canonical algorithmic method)
* Fix inappropriate uses of trunc, which caused several errors before the Julian Day epoch (39, 40, 42)
* Fix bug in `julian.leap` (43)
* Add official suppport and tests for Python 3.9
* Raise ValueError for attempted conversion of dates before the Mayan epoch

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* Adjust requirements to avoid a broken release (pymeeus 0.3.8)

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* Fix bug in Bahai calculations during Ayyám-i-Há (32). Thanks chiuczek
* Regularize variable names:

- coptic: rename `MONTH_NAMES` to `MONTHS`, rename `DAY_NAMES` to `WEEKDAYS`
- french_republican: add `MONTHS`
- mayan: rename `HAAB_MONTHS` to `HAAB`, rename `TZOLKIN_NAMES` to `TZOLKIN`

* Add some Jewish and Islamic holidays to `holidays`: `shemini_azeret`, `lag_baomer`, `tu_beshvat`, `tisha_bav`, `ramadan`, `ashura`, `eid_alfitr`, `eid_aladha`
* Add docs. See https://convertdate.readthedocs.io/
* Fix December 31st bug in `ordinal` (34)
* Add Indigenous Peoples' Day and Juneteenth to `holidays`, deprecate Columbus day
* Additional `format` methods for expressing dates as strings (37, thanks philosp)

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* Add `observed` argument to the functions for several American holidays (30)

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* Add orthodox/eastern Easter calculations and docs
* Add module for Armenian and Sarkawag regularisation
* Bump pytz requirement