Skip extra deps on OSX

[hmaarrfk]

Fixes #3894

imread should publish wheels to help us out :D

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
• Consider backporting the PR with @meeseeksdev backport to v0.14.x

[stefanv]

Looks fine to me; maybe a TODO to remove this again as soon as possible? But OK to merge.

[jni]

So much green! 😍

Well, imread should help all their users out, of which we are but one. =)