pip: bump edk2-pytool-library from 0.21.5 to 0.21.8

Bumps [edk2-pytool-library](https://github.com/tianocore/edk2-pytool-library) from 0.21.5 to 0.21.8.
- [Release notes](https://github.com/tianocore/edk2-pytool-library/releases)
- [Commits](https://github.com/tianocore/edk2-pytool-library/compare/v0.21.5...v0.21.8)

---
updated-dependencies:
- dependency-name: edk2-pytool-library
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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