pip: bump edk2-pytool-library from 0.21.5 to 0.21.6

Bumps [edk2-pytool-library](https://github.com/tianocore/edk2-pytool-library) from 0.21.5 to 0.21.6.
- [Release notes](https://github.com/tianocore/edk2-pytool-library/releases)
- [Commits](tianocore/edk2-pytool-library@v0.21.5...v0.21.6)

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

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

pip-requirements.txt

@@ -12,7 +12,7 @@
 # https://www.python.org/dev/peps/pep-0440/#version-specifiers
 ##
 
-edk2-pytool-library==0.21.5
+edk2-pytool-library==0.21.6
 edk2-pytool-extensions==0.27.5
 antlr4-python3-runtime==4.13.1
 xmlschema==3.3.1