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
)