tests: remove sys.executable from "required tools" In practice this doesn't appear to have been true for some time - we reference Python using the $PYTHON variable in all the tests now (which we have to for PyPy and Python 3), and I've been using ~/.../python.exe to test with tip of the cpython 3.6 release branch while working on manifest tests in Python 3 and everything seems to be just fine. The only real observable difference from this change is that I stop getting a warning about python.exe not being a thing on $PATH, which seems like an improvement.