It turns out you were pretty close to pinpointing the problem in
doctest, but didn't quite manage to identify which step was going wrong.
The problem was actually that even after __file__ was being set
correctly, the call to linecache.getlines wasn't being given the
__loader__ access it needed in order to retrieve the source code from
inside the zipfile.

The __main__.c thing turned out just to be a red herring caused by the
choice of class name in the test code (I changed it to be __main__.Test
for the checked in unit test).

Checkin versions:
2.7 = r67790
2.6 = r67791
3.1 = r67792
3.0 = r67793