to remove this package's dependency on zarr-python, we should do the following: - redefine all the codecs in [zarr3](https://github.com/zarr-developers/numcodecs/blob/main/numcodecs/zarr3.py) in zarr python itself. - replace all the codec definitions in [zarr3](https://github.com/zarr-developers/numcodecs/blob/main/numcodecs/zarr3.py) with re-exports from zarr python, if zarr python is installed. - remove zarr python as a dependency.
