v1.6.7 - Use ImportWarning on failure to generate ndarray.pyi

• #42, #43 -
  Use an ImportWarning rather than a UserWarning when a stubfile can't be generated and saved
  (e.g. due to a read-only filesystem or permissions error) so it only shows in CI and not at runtime.
• #44 - Zarr 3.0 changed a ton about how zarr works,
  so until we can adapt the interface, put an upper bound of <3.0.0 for zarr