Will be passed a match object corresponding to the interpolation key
we just found (e.g., "%(foo)s" or "$foo"). Should
look up that key in the appropriate config file section (using the
``_fetch()`` helper function) and return a 3-tuple: (key, value,
section)
``key`` is the name of the key we're looking for ``value`` is the
value found for that key ``section`` is a reference to the section where
it was found
``key`` and ``section`` should be None if no further interpolation
should be performed on the resulting value (e.g., if we interpolated
"$$" and returned "$").