WIP: Support calendar-specific cftime.datetime instances

[spencerkclark]

Since cftime version 1.3.0, the base cftime.datetime object can be calendar-aware, obviating the need for calendar-specific subclasses like cftime.DatetimeNoLeap. This PR aims to finally enable the use of these objects in xarray. We can also use this moment to remove cruft around accommodating inexact cftime datetime arithmetic, since that has been fixed since cftime version 1.2.0.

The idea will be to support both for a period of time and eventually drop support for the calendar-specific subclasses. I do not think too much should need to change within xarray—the main challenge will be to see if we can maintain adequate test coverage without multiplying the number of cftime tests by two. This draft PR is at least a start towards that.

• Closes cftime_range fails for base cftime.datetime object #4336
• Closes cftime default's datetime breaks CFTimeIndex #4853
• Closes cftime 1.5.0 changes behviour upon pickling : breaks get_clean_interp_index with a dask distributed scheduler. #5551
• Closes cftime.DatetimeNoLeap incorrectly decoded from netCDF file #8298
• Closes xr.cftime_range fails when using proleptic_gregorian calendar in cftime.datetime object #8941
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst