Bump backports-strenum from 1.2.4 to 1.2.8

Bumps [backports-strenum](https://github.com/clbarnes/backports.strenum) from 1.2.4 to 1.2.8.
- [Commits](clbarnes/backports.strenum@v1.2.4...v1.2.8)

---
updated-dependencies:
- dependency-name: backports-strenum
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>