[Cython] Slow traceback reconstruction in IPython between 0.15.1 and master

[Wes McKinney]

Just wanted to bring this issue to your guys' attention in case you
knew what was responsible for this:

https://github.com/ipython/ipython/issues/1317#issuecomment-3652550

I traced down the problem (with git bisect) to a seemingly innocuous
commit referenced in that GitHub thread. The issue seemed to only
present itself in IPython, so likely there was some problem with
inspecting the Cython frames for giving context around the full
traceback.

best,
Wes