Bump vegas from 5.5 to 5.6 in /extra_requirements (#489)

Bumps [vegas](https://github.com/gplepage/vegas) from 5.5 to 5.6.
- [Release notes](https://github.com/gplepage/vegas/releases)
- [Changelog](https://github.com/gplepage/vegas/blob/main/CHANGES.txt)
- [Commits](gplepage/vegas@v5.5...v5.6)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>