Update setup.py for Pyyaml v6.0.2

[Stacey-Ferreira]

In version 6.0.2, the 'run' function for class 'test' in the setup.py file contains the line sys.path.insert(0, 'tests/lib') which fails when running the python setup.py test command as this path no longer exists in 6.0.2.
As a fix that line would need to be updated to sys.path.insert(0, 'tests/legacy_tests') to conform with the file structure in 6.0.2.