There is this little warning message when the inferior python enters PDB
that seems scary for users new to python.el. It actually made me think
the mode (pdbtrack) wasn't working properly. So I wonder if this can be
fixed.

To reproduce:

1. open the attached file pybug.py in emacs
2. C-c C-c (python-send-buffer) on the buffer

After a second or two the echo area will show:

  pdbtrack: Traceback cue not found

I can reproduce this on GNU/L or OSX. This is due to the failure of
regexp match of python-pdbtrack-stack-entry-regexp. The contents of the
inferior python buffer is attached as in a screenshot.

Thanks.
Leo