pip: update edk2-pytool-library requirement from ~=0.21.5 to ~=0.21.6 (#238)

Updates the requirements on
[edk2-pytool-library](https://github.com/tianocore/edk2-pytool-library)
to permit the latest version.

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

Author: dependabot[bot]

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
 lcov-cobertura==2.0.2