We stopped support AtheOS in 2007. But, there are still references in the code:

$ git grep -i AtheOS
Doc/whatsnew/2.3.rst:Other new platforms now supported by Python include AtheOS
Doc/whatsnew/2.3.rst:(http://atheos.cx/), GNU/Hurd, and OpenVMS.
Lib/distutils/command/build_ext.py:        # for extensions under Cygwin and AtheOS Python's library directory must be
Lib/distutils/command/build_ext.py:        if sys.platform[:6] == 'cygwin' or sys.platform[:6] == 'atheos':
Lib/distutils/command/build_ext.py:        elif sys.platform[:6] == "atheos":
Lib/test/test_os.py:            # On AtheOS, glibc always returns ENOSYS
Lib/test/test_os.py:            # On AtheOS, glibc always returns ENOSYS
Misc/HISTORY:- Support for AtheOS has been completely removed from the code base. It was
Misc/HISTORY:- Support for BeOS and AtheOS was removed (according to PEP 11).
Misc/HISTORY:- AtheOS is now supported.
Modules/_cryptmodule.c:    /* On some platforms (AtheOS) crypt returns NULL for an invalid
config.guess:    i*86:atheos:*:*)
config.guess:   echo ${UNAME_MACHINE}-unknown-atheos
config.sub:     -atheos*)
config.sub:             os=-atheos
configure:atheos*|Linux*/1*)
configure.ac:atheos*|Linux*/1*)