Remove SKOptLearner because scikit-optimize is unmaintained

[basnijholt]

Description

Removes the SKOptLearner.

This continuously causes problems (e.g., https://readthedocs.org/projects/adaptive/builds/19128846/ and #333), mostly because https://scikit-optimize.github.io/stable/ is no longer maintained https://github.com/scikit-optimize/scikit-optimize.

It's not compatible with NumPy 1.24 which removed the type aliases (np.float, np.int, etc.).

Checklist

• Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• pytest passed

Type of change

Check relevant option(s).

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• (Code) style fix or documentation update
• This change requires a documentation update

[akhmerov]

This seems appropriate, but requires a version bump and explaining the removal in the docs. Maybe even raise a more specific import error perhaps?

[jbweston]

LGTM, but bump the version as Anton suggests