pylint: don't report __traceback_info__ as unused variable

this is a "magic" local variable used to add more debugging information in zope traceback, it's not supposed to be used directly so it was a false positive in our case.

This extends the default value of --dummy-variable-rgx that was defined to _$|dummy in pylint 1.4.4 ( https://github.com/PyCQA/pylint/blob/pylint-1.4.4/pylint/checkers/variables.py#L296 )