pip: bump edk2-pytool-library from 0.21.7 to 0.21.8 (#372)

Bumps
[edk2-pytool-library](https://github.com/tianocore/edk2-pytool-library)
from 0.21.7 to 0.21.8.

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

pip-requirements.txt

@@ -12,7 +12,7 @@
 # https://www.python.org/dev/peps/pep-0440/#version-specifiers
 ##
 
-edk2-pytool-library==0.21.7
+edk2-pytool-library==0.21.8
 edk2-pytool-extensions==0.27.6
 antlr4-python3-runtime==4.13.1
 xmlschema==3.3.1