remove zarr3 codecs

[d-v-b]

to remove this package's dependency on zarr-python, we should do the following:

• redefine all the codecs in zarr3 in zarr python itself.
• replace all the codec definitions in zarr3 with re-exports from zarr python, if zarr python is installed.
• remove zarr python as a dependency.

[d-v-b]

closed by #780