Uninstall step: set warnOnFailure to True

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Recent example: https://buildbot.python.org/all/#/builders/531/builds/2842

On "s390x Fedora Clang Installed 3.x", the git fetch -f -t https://github.com/python/cpython.git main --progress command failed with:

fatal: unable to access 'https://github.com/python/cpython.git/': The requested URL returned error: 503

The "git" step fails but has warnOnFailure = True. Problem: the "uninstall" also failed because the target/ directory doesn't exist. It marked the whole step as a failure, whereas it's expected that uninstall fails. It's not a bug in Python, just a network issue.