linecache: add support for ipython >= 3
When 0698cfe5 was made, we were using ipdb 0.8 and ipython 1.1.0. Now we are using ipdb 0.9.0 and ipython 4.1.2.
In the meantime, ipython introduced one more layer of linecache so we need to walk up the stack one more level to make w (using getlines) and l (using getline) pdb commands display proper tracebacks for python scripts and zodb components.