Fix coord_categorisation.add_weekday

[rcomer]

🚀 Pull Request

Description

See #4137

Explicitly make the coord_categorisation functions always use a cftime.datetime instance, so that tests pass with cf_units v2.1.5. I also removed the comment about Unit.num2date being deprecated. That comment dated back to the days when Unit was part of Iris, and I can't see anything in cf_units.__init__.py to suggest Unit.num2date is deprecated.

I also fixed a link to the black docs. This duplicates part of #4132, but it looks like the main tests in that PR can't pass without pulling in this change anyway.

I am on leave for the next week but happy if someone else applies any changes here if they are needed in the meantime.

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[trexfeathers]

Thanks @rcomer!

While there are many other uses of num2date(), they all appear to be happily working with the new default return of cftime.datetime already, so no need to add the same argument to all those calls too.