bpo-39142: Avoid converting namedtuple instances to ConvertingTuple. (GH-17773)

This uses the heuristic of assuming a named tuple is a subclass of
tuple with a _fields attribute. This change means that contents of
a named tuple wouldn't be converted - if a user wants to have
ConvertingTuple functionality from a namedtuple, they will have to
implement it themselves.

Lib/logging/config.py

@@ -448,7 +448,7 @@ class BaseConfigurator(object):
             value = ConvertingList(value)
             value.configurator = self
         elif not isinstance(value, ConvertingTuple) and\
-                 isinstance(value, tuple):
+                 isinstance(value, tuple) and not hasattr(value, '_fields'):
             value = ConvertingTuple(value)
             value.configurator = self
         elif isinstance(value, str): # str for py3k