Fix zarr.open default for argument mode when store is read_only

[brokkoli71]

• fixes zarr.load fails with ZipStore unless read_only=False #3042

If zarr.open receives a Store Object, its read_only value should be considered for the default of the mode parameter to avoid errors like in #3042

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[dstansby]

👍 for the fix, but

• zarr.synchronous.open needs the same fix (I think?)
• The docstring needs updating to say that the default mode is taken from the store if read only, otherwise defaults to 'a'

[brokkoli71]

Thanks for the feedback @dstansby !
What do you think of the changes?

[d-v-b]

@meeseeksdev backport to 3.0.9

[lumberbot-app]

Can't Dooooo.... It seem like this is already backported (commit is empty).I won't do anything. MrMeeseeks out.