Expose __version__ and ignore generated _version.py

[Nirkan]

• This allows users to query ome_zarr version using ome_zarr.__version__ similar to zarr & other packages.
• The generated file during the editable install named _version.py added to .gitignore to avoid accidental commit.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.08%. Comparing base (77e6d63) to head (3bdda37).

Files with missing lines	Patch %	Lines
ome_zarr/__init__.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
- Coverage   87.16%   87.08%   -0.08%     
==========================================
  Files          13       14       +1     
  Lines        1776     1781       +5     
==========================================
+ Hits         1548     1551       +3     
- Misses        228      230       +2     

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[will-moore]

Thanks for the PR.
docs build failed, presumably due to a bunch of warnings like:

WARNING: autodoc: failed to import module 'cli' from module 'ome_zarr'; the following exception was raised:
No module named 'ome_zarr._version' [autodoc.import_object]

[Nirkan]

Thank you for the comments.
Yes, probably because it is not able to find _version.py file.
Will try to add a fallback part to resolve the docs build error.