Extract ABSStore to zarr._storage.absstore

[joshmoore]

First step towards being able to assign CODEOWNERS
to simplify maintenance. This does not yet attempt
the rename to zarr.storage.absstore in order lower
conflicts with open PRs. Imports should remain un-
changed for the moment.

see: #764

[codecov]

Codecov Report

Merging #781 (d7d7f41) into master (efd94fa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #781   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          28       29    +1     
  Lines       10562    10568    +6     
=======================================
+ Hits        10556    10562    +6     
  Misses          6        6           

Impacted Files	Coverage Δ
zarr/_storage/absstore.py	100.00% <100.00%> (ø)
zarr/storage.py	100.00% <100.00%> (ø)

[joshmoore]

Re-opening to see if this goes green.

[joshmoore]

Note: this PR missed needs a commit to add zarr._storage to packages. Pushed directly as 2241a26

Without this commit,

          python -m pip install git+https://github.com/zarr-developers/zarr-python.git@master

fails with:

>>> import zarr
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/anaconda3/envs/z/lib/python3.9/site-packages/zarr/__init__.py", line 3, in <module>
    from zarr.convenience import (consolidate_metadata, copy, copy_all, copy_store,
  File "/usr/local/anaconda3/envs/z/lib/python3.9/site-packages/zarr/convenience.py", line 7, in <module>
    from zarr.core import Array
  File "/usr/local/anaconda3/envs/z/lib/python3.9/site-packages/zarr/core.py", line 32, in <module>
    from zarr.storage import array_meta_key, attrs_key, getsize, listdir
  File "/usr/local/anaconda3/envs/z/lib/python3.9/site-packages/zarr/storage.py", line 59, in <module>
    from zarr._storage.absstore import ABSStore  # noqa: F401
ModuleNotFoundError: No module named 'zarr._storage'