Bump immutabledict from 4.0.0 to 4.2.0

Bumps [immutabledict](https://github.com/corenting/immutabledict) from 4.0.0 to 4.2.0.
- [Release notes](https://github.com/corenting/immutabledict/releases)
- [Changelog](https://github.com/corenting/immutabledict/blob/master/CHANGELOG.md)
- [Commits](corenting/immutabledict@v4.0.0...v4.2.0)

---
updated-dependencies:
- dependency-name: immutabledict
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>