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*)

What about configure.ac?

"On some platforms (AtheOS)" in the comment in Modules/_cryptmodule.c is redundant. crypt() should return NULL on error, and it often returns NULL for an invalid salt.

Well, the configure.ac thing is some code that fails if you're running atheos.

New changeset 4eaf7f949069882e385f2297c9e70031caf9144c by Benjamin Peterson in branch 'master':
fixes bpo-31866: remove code pertaining to AtheOS support (#4115)
https://github.com/python/cpython/commit/4eaf7f949069882e385f2297c9e70031caf9144c