[v3] support for datetime and timedelta dtypes

[jhamman]

Zarr-Python 2 supported datetime and timedelta dtypes. This functionality has not made it into Zarr-Python 3 yet (in part because the v3 spec does not have established extension dtypes for these yet).

Example of the behavior for Zarr 2:

z = zarr.array(['2007-07-13', '2006-01-13', '2010-08-13'], dtype='M8[D]')
z
<zarr.core.Array (3,) datetime64[D]>
z[:]
array(['2007-07-13', '2006-01-13', '2010-08-13'], dtype='datetime64[D]')
z[0]
numpy.datetime64('2007-07-13')
z[0] = '1999-12-31'
z[:]
array(['1999-12-31', '2006-01-13', '2010-08-13'], dtype='datetime64[D]')

This issue tracks the development of datetime and timedelta support in Zarr-Python 3.