Implement `"sequential"` strategy for `BalancingLearner`

[basnijholt]

Description

Implement "sequential" strategy for BalancingLearner

Checklist

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

Type of change

Check relevant option(s).

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