Pandas dtypes in v2.0.0 will be free of nanosecond limitation

[tomvothecoder]

Thanks to @pochedls for finding this post.

Link: https://discourse.pangeo.io/t/pandas-dtypes-now-free-from-nanosecond-limitation/3106

With this improvement, we can potentially:

• Simplify logic with time coordinates (decoding, grouping, etc) by using pandas.DatetimeIndex
• Not worry about time coordinates outside the Timestamp-valid range (1678-2262)

There might be some cases where we still need to use cftime instead of pandas.DatetimeIndex including:

• Time coordinates that start at year 0 (e.g., climatology time coords)