Fix tests on Python 3.3/3.4 when run under coverage
Fix tests on Python 3.3/3.4 when run under coverage

As far as I understand, coverage somehow has the side effect of
instantiating the locals() dict of every frame, which then show up on my
backgrefs graphs, uninvited.  (Normally Python's fast locals optimization
keeps them empty.)

Exclude them explicitly.  This should fix Travis builds.

mgedmin/objgraph