remove the zarr dependency

[d-v-b]

we need to remove this package's dependency on zarr python.

To do this in a non-breaking way, we need to move the contents of numcodecs.zarr3 into zarr python. After this change, the zarr3 module will simple re-export code defined in zarr python, perhaps with a deprecation warning, since we should eventually not be re-exporting zarr python stuff from this repo. \

I see a sequence like this:

1. we add the zarr3 codecs to zarr-python
2. we remove the zarr3 codecs from numcodecs.
3. we release new versions of zarr-python (3.1.x) and numcodecs (0.17?) at the same time

After these changes, importing stuff from numcodecs.zarr3 will require zarr-python 3.1.x. This is still a breaking change for people who want to use the contents of zarr3 but can't upgrade zarr-python, but I don't know how else to solve this problem.

Any other ideas are welcome. We should prioritize getting this fixed.

[d-v-b]

cc @zarr-developers/python-core-devs for visibility

[d-v-b]

step 1 is achieved by zarr-developers/zarr-python#3376
step 2 will be #780, which is a draft until we do step 1