Climetlab

- Added support to python 3.10.
- Dropped support for python 3.6 (not supported by numpy anymore).

Changed
- When using GRIB files. ``to_xarray()`` does not remove empty coordinates anymore.
Use ``to_xarray(xarray_open_dataset_kwargs=dict(squeeze=True))`` to revert to
previous behaviour.

Removed
- ``DateListNormaliser`` has been removed. The same functionality is provided
by the decorator ``cml.decorator.normalize``.

*Added* for new features.
*Changed* for changes in existing functionality.
*Deprecated* for once-stable features removed in upcoming releases.
*Removed* for deprecated features removed in this release.
*Fixed* for any bug fixes.
*Security* to invite users to upgrade in case of vulnerabilities.