Add github-actions to `dependabot.yml`

[j178]

Type of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Documentation / docstrings
• [ ] Tests
• [x] Other

Checklist

• [x] I've run the latest black with default args on new code.
• [x] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• [x] I've added tests for new code.
• [x] I accept that @willmcgugan may be pedantic in the code review.

Description

Add github-actions to dependabot.yml