143 Retry for ml tests.

[chenkins]

Changes

• Add pytest-retry enabling retries for flapping ml tests.
• Run notebooks and examples with all supported Python versions.

Related issues

Closes #143

Checklist

• Tests are included for relevant behavior changes.
• Documentation is added in the flatland-book repo for relevant behavior changes.
• If you made important user-facing changes, describe them under the [Unreleased] tag in CHANGELOG.md.
• New package dependencies are declared in the pyproject.toml file.
  Requirement files have been updated by running tox -e requirements.
• Code works with all supported Python versions (3.10, 3.11 and 3.12). Checks run with all three version and are
  required to run successfully.
• Code is formatted according to PEP 8 (an IDE like PyCharm can do this for you).
• Technical guidelines listed in CONTRIBUTING.md are followed.