I took a peak at test.support.

a) I see that while many tests import test.support, or from test.support import <something> - not all tests use this.

b) I see that only 35 .py files in Lib/test have the string sys.platform.startswith, and there are 76 files that have sys.platform (so, there are roughly 40 files that have sys.platform without startswith).

I can start by adding _AIX to test.support and adding (as needed) from test.support import _AIX (and later _Linux, _Darwin) in the "35" files. If that seems to be working - and looking - proper the other 40 files could be added to the change.

Is this a good way to get started?