Fix tests with Pytest 8

[dstansby]

An attempt to resolve https://github.com/zarr-developers/zarr-python/pull/1671. The issue was installing the package in non-editable mode creates a copy of the package, and pytest gets confused about which of the duplicate files it should be importing. Installing in editable mode fixes this.

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/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)