Delete other/date_to_weekday.py as a how-to-use, not an algorithm (#5591)

* [mypy] Fixes typing errors in other/date_to_weekday

* [mypy] uses future annotation style for other/date_to_weekly

* date_to_weekday: new implementation replaces buggy original

* date_to_weekday: add examples from multiple of 100 years

* clean-up: runs `black` to fix formatting

* Delete date_to_weekday.py

Co-authored-by: Christian Clauss <cclauss@me.com>