fixes bpo-31866: remove code pertaining to AtheOS support (#4115)
fixes bpo-31866: remove code pertaining to AtheOS support (#4115)

We stop support this OS in 2007 with commit
19fab761b71a1687aee3415db3a937b5ce31975d. Let's finish.
pypa/setuptools,pypa/setuptools,pypa/setuptools