I figured  out what was causing the threading/subprocess problems (something in frame->f_code->co-filename was NULL), so the attached patch covers Python, C, and adds a test (as well as cleaning up the test_warnings file structure since it was old-school spread out in Lib/test).

I don't think Larry wants this going into 3.5 based on his previous comments (period, not just 3.5.0). Is that true, Larry? If so, would you accept a 3.5.0 patch to fix the warnings in formatter and imp to a more accurate stacklevel?