I started porting the patch to 3.x and it looks like I've uncovered another bug in sphinx:
$ sphinx-build -b doctest -d build/doctrees . build/doctest library/traceback.rst
..
**********************************************************************
File "library/traceback.rst", line 327, in default
..
Differences (ndiff with -expected +actual):
*** print_tb:
- File "<doctest...>", line 10, in <module>
? ^^^
+ File "<doctest default[0]>", line 10, in <module>
? ^^^^^^^^^^^
lumberjack()
(I added +REPORT_NDIFF to testoutput options for clarity.)
I even tried adding +ELLIPSIS to options, but this did not help. The same example works in 2.7.